I mostly have to add water (about a gallon and a half) to my turtle tank every 3-4 days due to the open top (no lid), heat generated from the heating lamp, and a fan that is blowing from the vaulted ceiling like 95% of the time.

All of my other tanks have lids, and maybe need 1/2 a gallon added to them once every 2 weeks or so.

I mostly have been doing top offs once a week, unless I’m gone where I try to fill it as high as possible before I leave.

Probably once a week, even with a lid on mine it seems to just disappear or evaporate.

I just do it once a week or so and that is usually fine.

This really also depends on the humidity level in the room. If it's winter, the furnace is probably on and that means dry air and so the tank might need to be topped off more often. In the summer, it's usually more humid which means less topping off to do.

Two weeks is supposed to be the maximum length. If the aquarium is heavily stocked up, then it must be weekly once.

I remember when I had my tank you would notice that the water had dropped within about a week and because of that I would always top mine up every 7 days or so depending on how far it had dropped within that time. I never left it any more than a week before topping it up though so that it wouldn't drop too far.

I usually top off my tank once a week. I just check how much the water level has dropped and add enough water to bring it back up to the original level. I agree with Shortie that it's important not to let the water level drop too far. This can cause stress for your fish and other aquatic animals. Plus, it's easier to add a little water each week than to have to add a lot all at once.

As Gilbert mentioned, evaporation rates can vary depending on factors like lid type and temperature differences. But keeping an eye on the water level and topping off regularly should help maintain a steady water level in your aquarium.

Note: Remember to always use water conditioner when adding water to your tank to remove any harmful chemicals that may be present in tap water.