A battery’s reserve capacity is a good guide. A 65 Ah battery lasts roughly two hours; with a broken belt, don’t keep driving.
Modern cars run servos, heaters, electric steering and many control units, all powered by the alternator. MSG Equipment says how long you can drive without it depends on vehicle type, engine, season and equipment.
The battery’s “reserve capacity” shows how long a charged battery can deliver 25 A before voltage falls to 10.5 V. For a 65 Ah battery the article puts this at about 100–120 minutes.
An important caution: if a broken drive belt is the cause, keep in mind that on some engines it also drives the water pump, so continuing may overheat the engine. MSG Equipment testers and benches are recommended for alternator diagnosis.