You can't have looked very hard!
Ops Pt A Ch 7 (FTLs) para 22.5
The longer of:
the length of the preceding duty, or
twelve hours.
If you can find the reference, there are some notes with it.
I'll draw your attention to note 3; where, if the rest is in company provided accomodation (ie a hotel down-route) the rest may be reduced by 1 hour.
Cheers and happy reading.