The more you use it, the more it asks of you. If I wake up to 300 reviews and then do them all, it'll normally go back to a hundred or so by the time I get back home.
I'm not sure if this is the right thing, but I just Skritter as much as I can. I rarely can get my lists down to zero. Once they get down to the 20s, more words get added to my lists so it is always growing bigger and bigger.
I wouldn't worry about the numbers. 300 something reviews seems like a lot, but then you realize it only takes about 5 seconds to write a character, so you will eventually go through them faster than what you are now.
This is just based on my experience. I'm not sure how everyone else uses it.