the duration of each turret is 1 or 2 waves, depending on the wave or the map, example: if there are 5 players, duration of the turret is 4waves because there are few zombies and ends faster the wave. when there are alot people, in mix wave all turrets stay one wave or less also. When the map is bigger all turrets stay a little more than a wave. Why I say this? Because all turrets can stay 2 waves (no more a few minutes, that I don't know how long it is!) when the first wave ends (in that time 30 seconds) armored can reload them but there is a problem when the map is bigger how can he go and reload all turrets? (if the way to reload them, is throwing ammo box near of them) because one turret was placed here, other there, far away of the another. With only one ammobox is impossible. Also, if armored throws ammobox to the turret, other players will not be able to recharge. With 2 waves of duration i think all people will able to buy a turret, and armored have enough time to reload them. But i don't know how he can reload them, pressing f, as engineer does to upgrade turret? Throwing ammobox near them? Or by pressing f, as we do to revive a partner? 
I love RotU 