slowsafe
60 mins is probs a good time, it was decided by someone a while back and gives plenty of time for the 'Plan' to go through the system.. dont forget its not just yours going through, FBUs deal with VFR, and IFR and these all take time.
I refer you to my previous answer, where I'm actually agreeing that a plan submitted via an FBU should be 60 minutes. I'm just stating that it's not a hard and fast rule for every plan, but depending on how it's filed - and I speak as someone who uses the Flight Plan system from
both sides of the coin and has worked in it for a considerable number of years
bookworm
So if you pitch up on Ground 35 mins after your EOBT, ATC has the right to have thrown your strip in the bin, but generally speaking they don't.
Ground might not have - but IFPS and therefore the UK host will have. End result is ground goes to ATCC for clearance, to be told "sorry mate, nothing in the system". Having been on the receiving end of that one it's amazing how fast you find your typing speed can be in an emergency
(And the time between that submission and the strip being available on the sector, and therefore a clearance being issued, was 90 seconds!)
30 minute dump is still true (not 15 as you so rightly summise) especially as the 15 minute change is a 'request' but a 30 minute change is a requirement for just that reason