There is another thread here on APUG where this question was previously dealt with, and a consensus was reached that the lights can go on very soon after the B&W print goes into the fixer. IMHO, the most entertaining post on that thread was made by Bob Carnie, who probably has made as many if not more prints during his life than anyone else on APUG.

IIRC, Bob's post is to the effect that in the past he always had waited much longer before the lights went on, and if he had known the information earlier, it would have freed up years! of his life (hope I have paraphrased this correctly).

I mention this just to highlight that all of us, no matter how experienced, can always learn something new.

Now if I could just learn enough to be able to print as well as Bob Carnie!