How long does it take for a morning after pill to work?

In theory, as long as you are not pregnant it will work as soon as the hormones are absorbed into you body, processed, distributed to the parts of the body where they work etc. Clearly the sooner you take it after unprotected sex the better it will be (never wait a few days like some do). Within a week or so you may/should experience period like bleeding indicating it has definitely done its job.

Try to remember that the morning after pill is no substitute for proper birth control and has a much lower protection rate against unwanted pregnancy than regular birth control methods. I've seen numbers here that suggest that as little as ~85% effectiveness is not unusual (or a one in six chance it wont work) if you were fertile when you had unprotected sex.

Good luck.