As much as your budget allows.
Maybe you've got an expensive lifestyle. You eat out three or four times a week. Buy a new car every other year. Take multiple vacations every year. That's all fine, but you may not be able to afford a lot for rent.
Take someone else who's more frugal. That person eats out once or twice a month. Buys a car every 8 or 9 year. Takes "staycations," not vacations. That person might be able to afford a lot more.
There's no magic figure--like 30% or 35%.