I do not think it really matters. If you are out of the main painful episode, you can start. Many people wait for many weeks and still get the gout attacks triggered with Allopurinol. I satrted it when I was in middle of it and it helped a lot. I did not get any gout attack triggered by Allopurinol though I started it in the middle of the pain and swelling.
So, in my opinion, wait is just wasting time. Let's see what others feel and say and then you can decide.