i don't think you can really set a time frame for when to tell your partner, but i don't know too much about this... i've avoided relationships my whole life. i would say you should either get to know them and let them know you have SA before you start dating, or wait until you feel safe enough. try to ease them into it, don't just be like "hey, guess what? i have severe social anxiety, and i'm in therapy and taking medication for it."I'll be honest, when me and my fiance started going out, I didn't have SA. It was a good two years into our relationship that I developed it.
Though I would tell them after we'd being going out for a few weeks.