If an opponent's clock is pulled, how much extra time is given? Today at the local tournament I noticed something strange. It was my turn, and my opponent's clock was running.
My opponent had pulled the time control of his neighbor, accidentally. We were playing two on each table, sitting four people on each table in a room. Some time had gone while our dear neighbor had lost time on the clock. Naturally, we did like this, pulling his clock. How much extra time would our neighbor get, the one whose clock had been pulled?
I called the judge and we tried to correct the mistake in a very improvised manner.
We did not know how long it had been this way. What to do?
First of all: unbiased sources might be available. A spectator might have noticed it or a player might have recorded clock readings on the scoresheet. DGT boards record the exact time, too.
Naturally, the player who lost time should get back exactly the time he lost.
Now three unreliable sources of information are available to the arbiter: the person who accidentally pressed the wrong clock and the two players on the other board. They should be able to agree on a rough estimate of the time elapsed. (Has it been five minutes or ten? You should be able to tell.) The arbiter should adjust both clocks accordingly, maybe adding a minute for the disturbance caused.
This is governed by article 6.13 in the FIDE laws of chess:

If an irregularity occurs and/or the pieces have to be restored to a previous position, the arbiter shall use his best judgement to determine the times to be shown on the clocks. He shall also, if necessary, adjust the clock’s move counter.