If you own a used vehicle and it's got rust and you can't afford to get it fixed how long will you keep using it ?

How long is a piece of string? Where do you live... climate... does it need a yearly inspection by government agency.... what make .. model... does it have a chassis.... .oh the list goes on.... but you are better off trying to stop the rust spreading... there are some good products out there that will 'cure rust'... stops the chemical reaction causing the rust... just painting over wont work... use a good wire brush and course grit emery cloth .. get vack to bare metal .. remove sharp edges ... the a good primer paint.... or even a clear varnish or lacquer and make a feature of it..

I've driven cars literally until they couldn't drive anymore so I'd say keep going

That's one nice thing about living in the desert , your vehicles don't rust. My truck is 25yrs old and not a spec of rust anywhere. The paint is a lil faded but I can live with that. Not uncommon to see 30-40-50 yr old cars or older still being used as daily drivers.