If you need more than 60 minutes to clear an EX trial (especially the realative easy ones from ARR except maybe Ramuh) then my guess is the group is too bad to clear the trial even with a 90 min timer.

If ppl make it to HWs (4th I think) EX trial Sephirot and still don't look up the mechanics before the fight I will just leave the instance if I get thrown there in mentor roulette. I think it's rude if you let other, prepared players wait until all the mechanics have been typed out to you. I am happy to help with specific questions about mechanics but that's it.

I don't think a 90 min timer will make a clear more likely than a 60 min timer, you will just see more replacements come and go.