When the Bible says "forever," how can we tell it really means forever? For instance, it is clear that God wanted the Sabbath laws to continue forever, based on these passages:


  Wherefore the children of Israel shall keep the sabbath, to observe
  the sabbath throughout their generations, for a perpetual covenant
  (Exodus 31:16).
  
  Every sabbath he shall set it in order before the LORD continually,
  being taken from the children of Israel by an everlasting covenant
  (Leviticus 24:8).
  
  It shall be a sabbath of rest unto you, and ye shall afflict your
  souls, by a statute for ever (Leviticus 16:31).
  
  And it shall come to pass, that from one new moon to another, and from
  one sabbath to another, shall all flesh come to worship before me,
  saith the LORD (Isaiah 66:23).


And yet systems in the OT do come to an end. Look at these two passages:


  And thou shalt gird them with girdles, Aaron and his sons, and put the
  bonnets on them: and the priest’s office shall be theirs for a
  perpetual statute: and thou shalt consecrate Aaron and his sons
  (Exodus 29:9).


We know from Hebrews 7:11-12 that Aaron’s office has been shut down:


  If therefore perfection were by the Levitical priesthood, (for under
  it the people received the law,) what further need was there that
  another priest should rise after the order of Melchisedec, and not be
  called after the order of Aaron? For the priesthood being changed,
  there is made of necessity a change also of the law (Hebrews 7:11-12).


Perhaps the words for perpetual and related words actually mean "a very long time," and not forever. Perhaps they are to be understood in a colloquial sense like when we say, "It took forever for the bus to get here," and they are not to be taken literally. 

There are people in Christianity who believe that we should follow the Law because it continues forever, even though it has been fulfilled in Christ and put aside, according to the book of Hebrews and other books. 

Under which conditions, or on what basis, are we to understand when the Bible really means forever when it uses such words?
While both עולם (olam) and Greek αἰών (aiōn) can mean &quot;eternity,&quot; they can also be limited to a particular duration of time or &quot;an age.&quot;
Olam Ha-Zeh and Olam Ha-Ba
The Jews typically divided time into two ages or worlds (עולמים): &quot;this age/ world&quot; or עולם הזה (olam ha-zeh), and &quot;the age/ world to come,&quot; or עולם הבא (olam ha-ba). In this particular context, we know that עולם couldn't possibly be understood as &quot;eternity,&quot; since there would not be a &quot;this eternity&quot; and an &quot;eternity to come.&quot; For more information, see the Jewish Virtual Library.
Contexts Denoting Finite Duration
There are certainly contexts in scripture wherein עולם simply cannot refer to &quot;eternity.&quot;
For example, in Exo. 21:6 (KJV), it is written,

Then his master shall bring him unto the judges; he shall also bring him to the door, or unto the door post; and his master shall bore his ear through with an aul; and he shall serve him for ever.
וְהִגִּישֹׁו אֲדֹנָיו אֶל־הָאֱלֹהִים וְהִגִּישֹׁו אֶל־הַדֶּלֶת אֹו אֶל־הַמְּזוּזָה וְרָצַע אֲדֹנָיו אֶת־אָזְנֹו בַּמַּרְצֵעַ וַעֲבָדֹו לְעֹלָם

Here, the English phrase &quot;for ever&quot; is translated from the Hebrew word לְעֹלָם (le'olam). But, could a slave really serve a master for eternity? Such is not possible, since every slave was mortal and would eventually die. In fact, what this tells us is that death frees man from the commandments (cp. Rom. 7:1-4). This is why a Jew who is born again is no longer under any obligation to keep the commandments of the Old Covenant (i.e., the Torah), including the Sabbath. Now, if by chance particular commandments were ratified under a New Covenant, then an individual would keep those, for it's an entirely different covenant.
In addition, Rashi says that the phrase לְעֹלָם in Exo. 21:6 does not mean eternity, but rather, עד היובל (ad ha-yovel, i.e. until the &quot;Jubilee,&quot; which occurs every 50th year; cp. Lev. 25:54). So, in theory, if a master had taken a slave on the 49th year and 360ish day of the year, and the 50-year Jubilee occurred a few days later, the slave was able to go free. Ergo, לְעֹלָם may only refer to a few days.

Many learned Jewish scholars have contended that the Torah of Moshe is eternal because various words used in reference to commandments seem to imply eternality. On the other hand, other learned Jewish scholars have contended that these words do not imply the eternality of the Torah.
In his treatise ספר העיקרים (Sefer ha-Ikkarim), Rabbi Yosef Albo (1380–1444) of Spain wrote,

יש מי שחשב להביא ראיה לנצחיות תורת משה ממה שנמצא בכתוב בקצת מצוות יזכיר לשון חוקת עולם ובקצתם לדורותיכם ובקצתם אות היא לעולם...אבל לא באו אלו הלשונות להורות על נצחיות התורה כלל, כי אפשר לומר שבהבטל שאר המצוות יבטלו אלו גם כן.

which is translated as,

There are those who have thought to introduce proof of the eternality of the Torah of Moshe from that which is found in scripture with a few of the commandments. They mention the expression &quot;חוקת עולם&quot; (&quot;a statute of an עולם&quot;), and לדורותיכם (&quot;in your generations&quot;), and אות היא לעולם (&quot;it is a sign לעולם&quot;)...But these expressions are not introduced to indicate the eternality of the entire Torah, for it is possible to assert that when the rest of the commandments are abolished, these will likewise [be abolished].

Sefer ha-Ikkarim, Book III, Ch. XVIII
Furthermore, he wrote,

מלשון עולם או עד עולם או חוקת עולם או לעולם אין ראיה, כי כבר ימצאו אלו הלשונות ודומיהן על זמן מוגבל, לא על זמן בלתי בעל תכלית.

which is translated as,

Neither is there any proof from the expression עולם (olam) nor עד עולם (ad olam) nor חוקת עולם (chukkat olam) nor לעולם (le'olam), for these expressions and their like are already found [to be] until a limited time, not an infinite time.

ibid
One example Albo gives is Pro. 22:28, in which it is written,

Do not remove the landmark of [עֹולָם] which your fathers made.
אַל־תַּסֵּג גְּבוּל עֹולָם אֲשֶׁר עָשׂוּ אֲבֹותֶיךָ

Clearly, this landmark could not have been made by their fathers in eternity. Rather, the גְּבוּל was a boundary made for a plot of land made by one's ancestors. When a father died, this land was inherited by a child. Hence, the commandments instructs one not to re-define the boundary, as it would infringe on another person's land (cp. Deut. 19:14). Thus, we see that עֹולָם in this particular context does not refer to eternity.