There is a kit available from the same manufacturers as those in the auctions which does all the tests that Caryl mentioned. It should be cheaper than buying them as single kits. I think mine cost roughly the price of two kits for all four tests + high ph.
I used to test every week when I first started, more so while my tank was cycling, mainly to see what was happening. Like Caryl, I don't test my freshwater tanks at all now, even the fry tanks get neglected! (this has probably got a lot to do with starting a Marine tank a couple of months ago, which gets all the attention and testing.)
There are arguments regarding accuracy of test kits, but I consider them useful mainly for monitoring any change, which is what you are looking for.