Personally for anything a gallon and under I do water changes every other day (or every day). I have used 0.5 gallons only as quarantines (people give them to me because "they don't use it since their fish died"
) and those I do 100% daily. Mainly because ammonia really gathers up fast in small tanks.
Speaking of water changes my 30 gallon is due ;)
For those who have test kits check something out. Mainly just ammonia testing. Got a couple containers? Fill one to be 0.5, and another 1 gallon. Each day put a very (couple flakes?) small pinch of fish food (or a couple pellets) which won't accurately be like fish poo but will be close enough.
Leave them for two days, test ammonia. Leave it for a week and test ammonia. Would you want to leave it for a month, only adding water to replace what has been evaporated? ;)