Because plan B is such a large amount of hormones it can have pretty big effects. Unfortunately not everyone reacts the same way, so it's hard to know "normal". Personally my experience was very very bad, very bad. Sick in bed for 3 days with nausea, cramps, and headaches kind of bad. Plan b can also have a large effect on your period -personally I had 2 periods in a month and they were some of the worst I've ever had.
I think as long as you're not bleeding through a pad every hour, you don't feel faint, and the period doesn't last more than 2 weeks you're ok. Talk to a dr or pharmacist if you're concerned about any side effects though. You know your body best so if something feels very wrong don't be shy about talking to your dr.
Plan B can cause irregular heavy periods, so it's probably not anything to be concerned about. Unlike Ducky, I haven't had very bad experiences with it. It usually causes a rather heavy period within a few days, and then you return to your normal cycle.
As far as how long it will last: That depends on how long your usual menses lasts, plus a day or so.
Thank you so much for the comments. I feel much better that I know I'm not the only one with this.