I’d say it depends on many factors.
Single or part of a couple. Living at home? Saving for a deposit for a house, paying a mortgage? Are you lower income with a need for a reliable car to get to work? Are you risk averse to running costs?
And most importantly are you ‘into’ cars? Would owning/running/driving a car be a thing/hobby for you or a status symbol only? I could we imagine a nice car that you took to track days, took to car meets, went on driving holidays making sense if it was work a little more.
For me in a middle aged house with 2 professional incomes and embarrassingly low bills but a desire to change lifestyle sooner rather than later and more than a eye to saving but still likes cars I would say about a third of the gross of one of us would be the max I would pay. Can’t imagine ever wanting to spend 1:1. There would always be better things to spend on. When I was younger on a smaller income I suspect the fraction might creep up more by necessity than desire.