As long as he/she/they want. No one is under any kind of requirement to write you a letter. No one gets paid to do it. They do it themselves, of their own free will and generosity.
As a college faculty member, I’m incredibly offended when one of my students says “Can you write me a recommendation, and I need it by ______”. Well, too bad. You’ll get it if/when I’m ready and willing to write it. I’m sorry if that sounds mean, but I do have a life outside of these walls and I’ll do my best to fit in your request when I’m able.