CAP 694 (for UK operations) says this:
6
Submission Time Parameters
6.1
The general ICAO requirement is that FPLs should be filed on the ground at least
60 minutes before clearance to start-up or
taxi is requested. The "Estimated Off Block
Time" (EOBT) is used as the planned departure
time in flight pla
nning, not the planned
airborne time. Exceptionally, in cases where
it is impossible to meet this requirement,
pilots or Aircraft Operators (AOs) should
give as much notice as possible, but never
less than 30 minutes.
6.2
In order to comply with the requirements of the Integrated Initial Flight Plan
Processing System (IFPS), FPLs for IFR flights should be filed a minimum of
60 minutes
before Estimated Off Block Time (EOBT)