What time is the "sixth hour" in John 4? "...and Jacob's well was there. Jesus therefore, being wearied with
  his journey, sat thus by the well. It was about the sixth hour." John
  4:6 ASV


It was translated as "noon" in the NIV


  "...Jacob's well was there, and Jesus, tired as he was from the
  journey, sat down by the well. It was about noon." John 4:6 NIV


But The Halley's Bible Handbook says that the "sixth hour" is in the afternoon (6 p.m.), because it's roman hour system


  "The 'sixth hour', v. 6, is the roman time, ie, 6 p.m." Halley's Bible
  Handbook page 474


In "Rome in Late Antiquity: Everyday Life and Urban Change, AD 312-609", by Bertrand Lançon, page 132, We found other time definition.

What time system John used?
The hour of the day for Jews was calculated according to 12 hours of daylight, from sunrise to sunset.

A Jewish source explains it:


  an hour in halacha is calculated by taking the total time of daylight of a particular 
  day, from sunrise until sunset,1 and dividing it into twelve equal
  parts. A halachic hour is thus known as a sha'ah zemanit, or
  proportional hour, and varies by the season and even by the day.
  
  For example, on a day when the sun rises at 5 a.m. and sets at 7:30
  p.m., one sha'ah zemanit, or proportional hour, will be 72.5 minutes
  long. The third hour of the day will come to a close at 8:37:30 a.m.


A Messianic source explains it (see also in the visual that the night hours were similarly divided into watches):


  A Hebrew Hour is defined as 1/12 of the time between sunset and sunrise, or 1/12 of the 
  time between sunrise and sunset. The only Scriptural reference to
  there being 12 Hebrew Hours in a Hebrew Day is found in John 11:9
  where  יהושע the Messiah asked a famous question, "Are there not 12
  hours in a day?"