I need about 1-2 gallons a week for my 55.
I guess it depends on lighting, filtration,open top, glass top,and climate.
When i get a larger tank i will definitely make an automatic topoff.
As dmc888 said, the amount of evaporation will depend on a lot of things. My biggest cause is probably that Arizona is so arid. We typically have a humidity around 10%. When I lived in Indiana I remember summer humilities in the 80s.
If you question is how often to top off I would suggest doing it as frequently as possible instead of waiting to add 2-3 gallons once a week. I try to top mine off daily.