You could try cost-benefit analysis.
Work out the cost of replacing the belt now.
Work out the cost of replacing the engine (or the car) after the belt snaps and the pistons hit the valves. Also include the wear and tear on your nerves if that happens at speed, and the non-negligible chance of it causing an accident. It's most likely to snap while you are accelerating hard, such as while overtaking (cue nightmares). Then there's the cost of having the immobile vehicle recovered to a garage.
Guesstimate the probablity of the latter event if you delay the cost of the former for too long. It's a fair assumption that if the belt had been replaced recently the vendor(s) would have used proof to support a higher price for the car. So most probably it's approaching or outside its service life.
For anything except a very old car that is used for fairly few miles per annum and that you expect has only another year or two left in it, you'll conclude that in the absence of a service history, it's best to replace the timing belt now. Safety considerations may convince you even for a temporary car of little more than scrap value.
BTW I think there may be some low-compression engines where the pistons can't hit the valves, in which case it's far less traumatic when the belt snaps. So if money is very tight and if you can establish that your engine is definitely in this category, the balance may shift a bit.