Ovulation is ideally supposed to occur midway through a cycle. Don't buy into this crap at all, as it is based on someone ovulating on day 14 of a 28 day cycle, and rarely is anyone exactly on that. That's why OPK's were invented. Even if you have a 28 day cycle there is no guarantee you will ovulate on day 14. Some women might ovulate on day 9, some wouldn't until day 20.
That being said, the period after ovulation until AF comes is fairly consistent from woman to woman. So when your cycle varies, there's still (for example) 13 days between ovulation and AF, just instead of ovulating on day 14, you might O on day 17. OPK's are useful for this, as you might not know your AF decides to be late one month til after the fact, and if you'd counted on O'ing on day 14, you could have missed it. (does this make sense??)
Now - spermies can technically love for up to 5 days, but whereas at the point of ejaculation there are around 250 million sperm, there could be just 400 left after 5 days. So it is the accepted theory to SWI every other day from about a week before expected O until day of. Once the egg is released it lives 12-24 hrs on average, so technically you could get preg if you only have sex the day of/after ovulation, but the sperm must be super awesome. It's usuallly better to SWI at least once in the 2-3 days after.
Until recently the theory of the 'every other day' was that the sperm count has time to build up on the off day. But new research has suggested the amount is negligible so as vino touched on I think one day, there is some evidence that every day is just as good if not better as every other day. More is more, so to speak.
Ahem. I am far from medically trained but I have friends in the medial profession and have read an astoundingly boring amount of material on this ... Does this help anyone?!
I have enjoyed a good waffle anyway 