We'll be there Tuesday May 17th - Friday May 20th.

They open at 9am on these days & we aren't planning on doing any tours (we'll just set aside a whole day to focus on everything Vatican)...

So, I have 2 questions:
-What time should we get in line...8am or 8:30?
-How long will we be waiting in line if we get there at 9 or 10am on one of these weekdays?

You're talking about the Vatican Museums, right? I was in Rome in February. The day I decided to go to the Museums I got up really early and joined the line at 0730. At that time the line was already 60-70 meters long.
Another day I was walking near the vatican at around 0900 and saw a waiting line to the museums that ended at Piazza del Risorgimento!!!.
Conclusion: Go (very) early.

GO, A EARLY AS POSSIBLE, NO LATER THAN
8:30 A.M

I'm not sure I agree with that idea of going so early. We went a little over an hour before opening, so of course we waited in line an hour plus about a half to get to the front of the line. Those who came after it opened were pretty much in the doors within an hour. Sure the lines are a lot longer after they open -- but they move all the time -- you don't stay in the same place for an hour. I'm convinced that I'd rather be in a moving line for one hour and then get in, than be in a stationary line for an hour then move for half an hour.

I guess we were really lucky. We went to Rome just a few weeks ago. The lines for the Vatican weren't bad for us. We went the Basilica and the Pope's tomb first around 0930 with no real waiting time. From there, we went over to the Vatican Museum with no major waiting time there as well. The longest line we stood in was the line for special "VACANT SEE" stamp. We waited almost four hours for that. We never waited near that time for anything else we went to visit.

Christmas 2003 got in line at 8AM, inside by 8:35.

I don't agree with the strategy of going early. Last year in March we went to the museum a weekday( I think it was Thursday) around 1PM. There was NOBODY in front us either buying tickets or going in. We just bought the tickets and waltzed right in. I couldn't believe it! We did this on the advice of the limo driver that brought us from the airport to the hotel. He told us that the best time to go was around lunch time and he was right!

"Last year in March we went to the museum a weekday( I think it was Thursday) around 1PM. There was NOBODY in front us either buying tickets or going in."

Give the man a cigar. The smart people don't bother trying to fight through the tour groups first thing in the morning. They show up after lunch and waltz in.