When things are going well, we test every 3 months.  With dose changes, we usually test after about 4-6 weeks.  If we are particularly concerned about something, we have tested as soon as 2-3 weeks after a change. 

When we test urine, we test for: 17-Ketosteroids, Pregnanetriole, and Creatinine.

When we test blood, we generally test for: 17-Hydroxyprogesterone; Androstenedione; Testosterone; ACTH; Also, because my son has an issue with Precocious Puberty, and is on Lupron, we also test for Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH).  My son is not a Salt-waster, so we do not normally run Renin. 

Actually, as I am writing this and looking back over a chart that I've kept of my son's tests and results, I realize that, in about 2 years, we've only waited 3 months in-between tests once.  Generally, we have had much shorter intervals in between tests.  Part of that is because we have had somewhat of a checkered past---changing doctors back and forth, and starting and stopping different medications within a short period of time---so it has been necessary. 

We are lucky that our doctor is a very good listener.  If I feel that there is a problem before we are scheduled to do the next test, I let him know, and he usually tells me to run the test sooner.  We have the tests run here at the local hospital, they send him the results, and he lets me know as soon as he hears.  One thing he does is give us test orders that are somewhat open-ended so that we can run them as frequently as need be.  For example, he will usually indicate on the script, something like, "One-year standing order; every 4-6 weeks."  Maybe you can ask your ped. about doing the same.