It depends for me, i activated god mode earlier but i couldnt before, but even with god mode on i still died after a period of time.Weird, I just tested and its still working fine.
if you finished a mission and you going another one,you need again to re-activate god mode.but god mode is unstable meaning you can die in a car,hited by rockets too.Some time work fine , some time can't active and some time only active ammo .