How long will my car run without the battery charging?

So, my car has died. Basically, the timing belt has snapped and the alternator is not working. The engine itself runs, but the battery will go flat, as it is not getting charged. I have it parked outside my flat and need to get it to a garage to get fixed. If I charge the battery up, how long will the battery last before the engine dies?

I should probably mention that it is a petrol (not diesel) engine. Oh, and I’ll turn off all the lights, a/c, heating, etc. I know this is quite an open ended question, as it obviously depends on the speed I’d be driving, but I really have no idea. Will it die almost immediately, or would it keep going for say 10 mins?

[...and yes, it serves me right for buying a french car.]