Before my 3.5 (it's actually a tad less) cycled and had bulk plants, I changed 50% every 3 days, religiously. 4 days is about how long it took to show ammonia on the liquid test kit (Cleo was smaller back then, too!), so I took it back a day to keep it cleaner.
Once a week, usually, I'd do 100% - but once it was full of plants and cycled I've had no ammonia show for a week at a time (testing it every day! still, I didn't like doing this..). I now change the water 25-30% twice a week and get 0 ammonia every test.
I guess it depends on the tank. For unfiltered, no plants, I'd probably stick to "change it the day before it's due to read ammonia" and test frequently anyway.