I personally knew when I was done when I had exhausted most of my prep material. I used the OG, the OG verbal, Kaplan 800, Kaplan Premier and PowerScore CR Bible.

But that wasn't my only criterion, of course. I was aiming for a 90% hit rate on all sections (quant was my strong point, so I didn't even practice for that one except for answering questions on this forum) of the verbal. I had a pretty good start with RC, since my accuracy was about 95% when I'd started prep. Weaknesses were SC and especially CR. When I got to over 90% for each section, then I was sure I was ready for the test.

In the end, it's all up to what your initial goal was. Mine was pretty damn high, so 90% was my target. But if you're not necessarily aiming for that 99th percentile score (i.e. you'd settle for something less than that), you might want to set a lower standard.

All the best with your prep!

I had a fairly simple way of knowing when I was ready. When I was able to look at just about any question (>90%), analyze it in my head within 1 minute, parse/handle the data all in my head without taking any notes, and come up with a quick "strategy" to attack the problem, I knew I was good.

Why? Because if I could do that that fast inside my head then I would certainly be able to do it with pen and paper with written notes and more thorough thinking.