If you have a regular 28 day cycle then you are fertile roughly from day 12 to 16, but as sperm can live for up to 3 days you could get pregnant from day 9.
The kits are a bit pricey so it helps if you have a rough idea of when you are likely to ovulate (ie, by doing the temperature thing each morning). They are fairly similar to pregnancy tests in that you have to wee on them and you get a little blue line if you are going to ovulate in the next few hours. I used them when I concieved my baby and it worked first time so I would definately say it's worth doing, especially if it's taking a while. Hope it works out for you.