Meg jsut went through her first heat, and she's 4 lbs so we've been hesitant to spay her. It lasted for about 3 1/2 weeks for her. The frustrating part was the bleeding. I found that the little undies from Build-a-Bear ($4 each!) worked perfectly. I cut up a thin sanitary pad and put it in the undies. I attached a bell to her collar so that I could hear when she needed to pee, especially in the middle of the night. You can also keep her enclosed in a tiled area, but I felt bad about doing that. The bleeding lasted about 1 1/2 weeks for Meg.
Meg didn't have any spotting during her fertile time. It wasn't a problem at ALL for us since she is never outside without me being next to her. I let her play (on a leash) with other small, leashed dogs in the neighborhood, and none of them got close to mounting her, even though she was stickign her little tooshie up in the air (what a slut!
). Most of the dogs were fixed, but one puppy was not yet fixed. Of course, to be on the safe side, keep her away from all unfixed dogs. We were extremely vigilant in watching Meg while she was outside. All it takes is a second!!! (Well, actually I don't konw how long it takes, but I imagine it happens fast since so many dogs get pregnant "accidentally".)
It was hard to know when she was totally finished with her heat. The biggest change was that her nipples and vag went down to their normal size. They had swelled significantly during her heat.
Good luck! It's a pain to deal with a heat, but when the alternative is putting your dog under, I guess it's worth it.