The endurance of the T7 was 45 mins and you could count out 10 mins for start-up, taxy out and taxy in. This left you 35 mins, and now you had used (say) 5 mins to get to the upper air (where you did all your exercises).
I recall reading a comment from some ex-bomber baron posted to CFS to fly the Meatbox:
"When I'm down to 30 min endurance, one radio and no navigation aids, I normally declare an emergency - rather than ask for take-off clearance!"