In theory it’s 6USG/hr giving you 4 hours endurance with 24USG tanks. I prefer to use 7USG/hr, giving me 2.5 hrs + reserve. (If anyone can stand more than 2.5hrs in a 152, they’re a better man than me).
The ones I fly tend to use around 6.5USG/hr (local trips <3,500’, 90KIAS, fully rich) so 7 is a nice round number to use for planning, and gives you a decent margin for error.