It varies, it can be as soon as you come off it or it can take months its just the same as if you tried to conceive after using e.g.) condoms and then stopped using them.
It took my wife about 2-3 months, but the doctors said it could take about 6 months plus for the body to normalise and be ready after the pill. She was on it for about 12 years.
Everyone is different though, so there is no particular rule.
as quick as the same day i suppose if its possible to fall pregnant after missing a pill.
Mini-pill is supposed to take less time to leave your system than the combined pill as well.
It's taken me 6 months to get pregnant after coming off the Pill but it does vary. One of my best friend's became pregnant more or less as she came off it and another mate took 4 months.