I have growing out seedsman - white widow, it's the closest thing to black widow, I have tried GH WW and wasn't as good.
I would always go with a week or 2 than what the seed companies specify, as usually that's just an average, and there are also different phenotypes, so which may take longer, some which maybe shorter.
Or, you could check the trichs with a 30 x jewlers loupe, and keep checking the trichs when you want to finish.
I would say, 8-9 weeks is about right for the WW, however you do get the odd 1 that finishes on 7 weeks, as jrk said.
All in all, it's a pretty good high too, it's in my top 5 smokes.
Good luck dude.
Peace