Ive got a 4' by 15" by 12" tank and have had it for 5 years. I still have 2 of the original 5 white widow tetras I bought when I started, going strong!
Anyway I am a bit lazy I'm afraid and sometimes change some of the water at more than monthly intervals and I very, very rarely get deaths and had no diseased fish I can think of.
If you have a good filter and tank is not overstocked you can get away with it a bit.
When I am trying to be good I change about 30% once a month.
Clean and check/change partial filter media about 4 monthly.