Keep an eye on the monitors in the lounge, as they usually announce the gate number at T-90. Since your flight leaves at 11.50, the boarding pass will probably have boarding written at 10.50, but in reality they begin boarding 30 minutes before departure like most other airlines. So you should be fine turning up at the gate 45 minutes before departure. I haven't had a gate change at the 5 or so times I've flown through Istanbul in the past year, so I doubt you'll have any issues with that...