For me, it was immediate. I came off the pill, 28 days later I had a period and the next month I got pregnant. I was very lucky. I don't know anyone else who can say that.
As for when you'll get pregnant- you can only get pregnant when ovulating, so when the egg is released from your ovary and the 12-24 hours following that. You ovulate on average 14 days BEFORE your period. So if you have a 28 day cycle you will usually ovulate on day 14, where day 1 is the first day of your period.
You can work out when you ovulate a number of ways, the easiest is by cervical mucus- days 1-5 or so will be blood (your period) then snot like, then watery, then like egg white, where if you get it on your fingers its stringy and stretches between them. Egg white cervical mucus usually means you are about to ovulate and lasts about 3 days. Then it's snotty again, before it goes a bit grey and glue like then you dry up, then your period starts. To test cervical mucus you need to get it from the source (i.e. up your foof, not on your knickers).
You can also use basal body temperature and ovulation predictor tests to predict ovulation. I found cervical mucus by far the easiest and most stress free way.
Making babies gets you really I touch with your body!