The answer to this question will vary depending on the woman taking the pill. Other medications, how regular the menstrual cycle is normally are only a couple of factors. When you take the morning after pill the best thing to remember is to expect anything.
The morning after pill is really just an extra strong version on a regular birth control pill. Its job is to prevent the ovaries from releasing an egg. It doesn't matter if you take birth control pills all the time or you've never taken it, side effects can range from almost none, to an irregular cycle for months, oh yeah and vomiting, cramps, and nausea, but those shouldn't last more than a day or two. Even if you have had a regular cycle of a 4 or 5 days, once you take the morning after pill you could all of a sudden have a 12 day period, and then next month have a 2 or 3 day period. Things will go back to normal, and if you're really concerned visit your doctor, what you don't want to do is blame the pill for something more serious and avoid getting checked out.
For more information on the morning after pill and what it is check this site out:
http://www.netdoctor.co.uk/sex_relationships/facts/morningafterpill.htm
