Well if the alternator dumps while i'm driving how long can I go on just the battery?
Chances are, the alternator has already 'dumped' and that is *why* the light is on. SO you are driving on borrowed time now.
Anything that draws electrical power is going to reduce your drive time. Think Apollo 13 - you need to shut down everything that is not absolutely required. Use the turn signal stalk to disable the DRLs, turn off the radio, turn off the nav, do not plug you cell phone charger in, make sure the dome lights are off, do not turn the headlights on (read: don't drive at night).
I would wag that you could go 30 to 60 minutes before a fully charged battery dies.
On older diesels you could drive indefinitely without an alternator or battery. You snipped the fuel shut off plunger off at the injector pump and once you got it started (push starting...) it would run until you stalled it (the key would not shut it off, as it was all mechanical at that point).
The newer diesels use electricity to power up the fuel pumps, ECU, injectors, crank timing sensors, etc. The electrical load is pretty high even with the non-essentials powered down.