As 1077 mentioned above there is no "scientific formula" for cleanings, filters etc.
Your best bet is to get the filter for the size of the tank you are getting(they are labeled) so you can jus tplay it safe there.
If you want to be safe with amonia levels then you need to find the best amount of water to take out and how many times a week you do that. The best way to do this is by ration: The smaller the tank the more cleanings it needs and the more fish you have the more cleanings you need.
Examples: I have a 10 gallon tank with a single male betta in it and cleaning it once a week is fine since I have a filter. It was not cycled when I got the guy and I have had him for only a month and it has been fine. Just be diligant in the beginning and check the amonio and nitrite/trate levels twice a week.
The safest route, ESPECIALLY if you plan on getting a large community of fish is to cycle the tank. Yes it takes a few weeks and yes it can be a very annoying process but you do not want to spend so much time, money and care for a bunch of fish to have them die on you. Your best bet on this route is to use plants to cycle the tank and use some water from the tank you get the plants out of. That is how I always cycle my tanks and it works wonders(cycling with plants and the proper lighting/chemicals only take two-three weeks for me).
Hope this helped
edit: Also I would like to add if you plan on getting a Snail then plan on cleaning your tank even more. Those things produce more waste then they eat >_> They are good for an established tank(I usually wait a month or two to get a snail because of the waste >_> )