The first month I gave up the pill (we were trying for our 2nd baby) I waited 6 weeks for a period. I was sure I was pregnant, but I wasn't.
It can take your body a little while to get back to normal after stpooing the pill, mind you, I got pregnant the next month, so although your periods might not be regular yet, you may still ovulate, so if you don't want a baby, you should be very careful about using alternate protection. Also- don't rely on timing sex for "safe" times of the month- if you have just come off the pill, your ovulation might be iregular too and might not be when you would normally expect to be- I concieved two days after the period that took 6 weeks to come ended, according to the theory of ovulating 14 days before a period, I should have ovulated more than 11 days later.
You may even ovulate more than once in a cycle.
Use condoms.