How long?

I'm not TTC but I do you natural family planning as contraception so I figured you ladies would know the most to be able to help me  

How long does an egg live? Most places I read say 12-24 hours so that's been the info we've been going on to NOT get pregnant. So I got a pos opk on thursday night and we just had sex this afternoon thinking that 36 hours max from a pos opk for ovulation to happen would be Saturday morning and that an egg only lives max 24 hours so Sunday afternoon we should be good right?! Except now I'm reading things that say that it can live up to 48 hours and some places are even say 5 days (seems a little long to me   ), so now I'm wigging out a little.

I've heard 12 to 36 hours....