I still have the box, and original booklet for all the tests, but I've had to replace solutions many times. It cost $16.49, but that was a long time ago.

I'm a bit of a test nut. I probably test more often than I need too, but I like to do tests that all come out good!!! It kind of keeps me sane. ;-)

I also keep a log for each tank, that way if I don't have time to test, when I do, I can look back and compare to the last test done.
This keeps me from freaking out sometimes.

When I set up a tank I test pH, ammonia, and nitrite daily.

After that, I kind of test by the size of the tank.
The smaller the tank the more often I test.

I have a very small tank with four fry in it right now, so I test the pH and nitrite daily on that one.
Otherwise, every couple of days for my 5 gal., and at least weekly for my 40 gal.

I try to do a count on all my fish daily, and try to find time to observe them each day as well.
I also ALWAYS check on the temp. of the tank each day when I feed them.

I have a female Red Velvet Swordtail in my community aquarium who is very succeptible to changes in water conditions for some reason.
So I watch her.  If her fins are clamping, other than when she is swimming, or if she slows down, or even acts a little funny, I run for my test kit.

I have 3 female guppies in my 5 gallon right now, and they are pretty hardy girls. "Fertile Myrtle",(The mother of almost all of my fry) and "Aunt Stell" are getting on in guppy years, and don't give me a lot of help monitoring, ie. nothing seems to bother them.
"Little Lulu" is much younger(4th generation), and is pregnant right now, so I watch her. All she really likes to do right now is eat. So I just watch to see if that changes.