Checked mine and it was at 14.25 hours. Was a total surprise when it happened. Not sure how different schools deal with it. I was booked for a lesson with the instructor that I had flown most of my exercises with. Got to the flying school to be told he was ill but it was ok because the CFI would take my lesson. This worried me a little as I had flown once previously with the CFI and I wasn't exactly thrilled with his style but whatever. Goes up and does 4 or 5 dual circuits. On last downwind leg, CFI says make this a full stop landing. Puts in downwind call and continues with no problems. Vacates runway at normal taxiway at which point CFI says to stop at a hold on the taxiway after normal post-landing checks. As we stop, CFI calls ATC and says student going back up with 1 POB for first solo. He tells me to call them when ready for departure, go do one circuit just like those you've just done, then says drop throttle back to idle while I get out.
Worked really well as it meant no time for nerves. I think it was climb rate after takeoff that made me realise I was on my own but in the circuit you don't have time to get nervous, too many things to remember and there was quite a bit of traffic. I had taxied back to the clubhouse before it kicked in, where I was met by various members including my normal instructor who had apparently been watching from the car park.
In the whole of my training flights I think it is number 2 in my memory behind the QXC.