If your primary drogue charge doesn't fire or doesn't break the shear pins, you're going to start falling pretty quickly. By that five seconds, you'll probably be going close to 100 ft/sec (not counting any horizontal velocity you may have had). A good drogue should be able to handle that, so can a premium main chute. Nevertheless, 2 seconds is probably a pretty good maximum for your backup drogue; the less stress you put on them, the better.
The Eggtimer and Quantum altimeters can go up to 9 seconds, but we generally don't recommend going that late for a backup drogue. Note that the Quantum can also optionally fire the main chute if it detects that you've fallen for a specified time at a specified velocity; we recomment 100 ft/sec for 1000 ms (one second). If you get there, your drogues have failed completely, so that "FailSafe" feature will prevent you from coming in ballistic.