Engines differ and some can be leaned all the time, even on takeoff so there is no hard and fast rule you just have to know an engine and its peculiarities. 3,000' msl would be a good time to do it if it hasn't been done by then. Make sure you have setting the mixture back to full rich before getting into the traffic pattern on your checklist, especially when there is any doubt about how your engine will run leaned out. Back when I did my very first solo cross country I descended to the pattern at the destination without going back to rich and the darned if the engine didn't cut out. I knew what was wrong and managed to airstart it right away, and that airplane was more sensitive than most, but you could get in trouble if you didn't richen the mixture coming back down from altitude.