The size and frequency of the water changes should be decided by a test kit, not necessarily the clock. For example, a 50% water change reduces toxins by a half, a 25% pwc by a quarter, etc... The goal we shoot for in a fish-in cycling tank is to keep ammonia and nitrIte at or below .25ppm at all times. So if the level is 2ppm...a 50% change will bring it down to 1ppm which is still very unsafe. Personally at that level I'd do a 75% change (with temp matched, conditioned water)...then wait 20 minutes or so for the water to circulate and do another large pwc (50%+).
There's a lot of variables which decide how quickly toxins are spiking. Things like bio-load, phase of the cycle, how much the fish are fed, how well the tank is cleaned, etc...so there really isn't a set time to change...you should always let your test kit dictate when it's time to change water.
The only saving grace is if you're using a water conditioner like Seachem Prime which temporarily neutralizes ammonia and nitrIte. I believe they'll detoxify up to 1ppm for 24-36 hours...but that still requires daily water changes to re-dose it and make it effective again. It's also never a good idea to put all your faith in a bottle when you've got a faucet of clean water which can remove the toxins in a natural way. Prime really is a safeguard to buy your fish time between water changes.
Here's a good article to check out which covers how to deal with toxin spikes and fish-in cycling-
http://www.aquariumadvice.com/articles/articles/124/2/-I-just-learned-about-cycling-but-I-already-have-fish-What-now/Page2.html
* You adjust the flow on the Aqua-stop valve of the Fluval...but I'd keep it full power on a tank your size.