With my filtered 1 gallon tanks I did 50% twice a week and 100% once a week. So 50% water change monday, thursday, and 100% water change and gravel rinse on Sunday. I upgraded to 5 gallons because all those water changes were stressing out my fishies 3 times a week. Without a filter, the water changes would be 100% every other day. If you want a filter for a 1 gallon tank, I would suggest tetra 3i
. It worked well for my 1 gallons and they last a long time. The only reason I replaced my filters is because the new ones they sell now have O rings. The small size doesn't take up much room so the fish don't fell cramped. I stuck my filter in one of the narrow sides of my old corner aquarium
Also, a bit of advise. If you do go with a 1 gallon setup, you may want a bucket of water so you can make the water you need for all your water changes for the week and a little extra for any sudden ammonia peaks. That way you just dump out the amount of water you need to replace and then scope out the clean water from the bucket. The bucket only has to hold 2-3 gallons, so it wouldn't be big.
1 gallon set ups work, but they are more work than people realize. :)