While you can put a multi-meter on the alt to see what voltage it is putting out, your dash gauge is close enough. 10volts is well below the 14-ish it is supposed to be churning out. Plus, the rest of your signs, your alt is dead.

As for how long to drive on battery power, you have a while. But...you don't know how long that alt has been working on not fully charging the battery. You get a bit more leeway with the deep cycle battery, but it is still not a great idea to kill the battery. I got 6 hours out of my batts, with headlights on, last weekend when my alt died, but I have two and it is not an Isuzu. It also died in the driveway when I got home.

Another option is for you to find someone to rebuild your alt. Just look in the yellow pages for alt repair, auto electrical places ect. They fixed me up this week for ~$100. Not bad compared to the other options.

Good Luck,
Michael