I think it depends on several factors, like the person who is speaking's age, if they have ever used they/them for a single person liked this before, how long they have known you, ect.seems like plenty of time for somebody to get used to? Or no..
For example, my stepchild is gender neutral and has been for maybe 4 or 5 years now. I easily switched to they/them within a month. Their dad, who has known them longer took about 3 months. Their grandparents STILL make mistakes many times daily and misgender more often than not.
When my stepchild first came out their dad and I sat around having conversations practicing using they/them appropriately. It really helped.