From what I've read...it depends on time of season. It seems like 4-5 weeks for most, and then I think the wintering bunch can last two months or perhaps even longer (I think one of the "My Monarch Experience" threads is about a butterfly that someone kept as a pet for about 2 months).
I'm dealing with two monarchs, one which has died and the other appears to be dying. They were both healthy upon birth (at least, from what could be observed physically). Both were able to fly.
The first to hatch I think was a screw up on my part because I thought having flowers was enough. I watered too, which surely created puddles etc., but she just wanted to cling to the screen facing the outside world. Twice I managed to get a hold of her and have her drink from the flowers...but other than that, no luck.
This morning I came looking, spotted one, but not the other...I opened up the cage, and there she was on her side on the floor. She was alive at that point, I put her on a flower and she drank. But she simply could no longer stand upright. I gave her a spray of water (indirectly) and she sort of fluttered back to the ground. I went to the grocery store and bought some Juicy Juice (something I read of here on MonarchWatch), but when I got back I was too late. She didn't want to drink or anything. I even helped uncurl her proboscus, but she was done for. I put her in the freezer.
Now, the newer hatchling, only a day old, is tired and lathargic. I don't know if it's just the time of day (1:30 pm) or if they get too much sunlight here on the patio (from about 7 am til about 12-1 pm for direct sunlight). At least he still has a bit of fight in him. He's not drinking anything (Juicy Juice or the flowers I put him on) but at least he's making an obvious effort to hold onto whatever I put him on (seems to enjoy my fingers most of all).
I wish I knew what the hell was going on....