Most qualifiers are 5 to 8 minutes long, just depends on where you race.
As far as mains, again it depends upon where you race. Typically, i've seen club level races with C and lower mains usually running 10 minutes. B-Main is usually 15 or 20 minutes and A-Mains are typically 20-30 minutes. A-25 mins B-15 mins C & Lower-10 mins is typically what I've seen at the club level. Some special club races might see longer A-Mains (45-60 minutes) and maybe longer lower mains as well. Coming from Electric, you will really enjoy the long nitro mains!!!
As far as pit stops, it really helps to have a buddy with you, but not required. Most places i've been to have someone more than willing to help you out. It really works best if you have a buddy that runs a different class from you so that you can pit for each other. That being said, i've gone to plenty of races myself and haven't found a problem with getting someone to pit for me. You just gotta ask someone!!!
That's about it, I guess. Welcome to the wonderful world of Nitro!!!