im irregular as well so I basically start 5 days into my cycle. If you can, try and do one a couple of times a day. The hormone they pick can disappear in a matter of hours. The idea is, you will start to see a faint line which should get darker as you near OV.
A perfect one will get as dark as or darker than the control line. But as many ladies on here will tell u, sometimes they dont get that dark.
Hope that helps!