I say once a month changes should be fine.
From the time I started with aquariums in 1960 (age 12) until 1997, I kept lots of tanks going all the time. So for 37 years ..... I kept tanks going fine with clear water and no diseases with monthly 20-25% water changes and gravel siphons. And NO test kits ...... you only took action if you got a disease, water was cloudy, or algae overtook your tank.
So I retired and moved from KS to FL in Nov 2010 and started back with the hobby with a 38 gallon tank with 6 fantail goldfish + pleco, 300gph HOB filter, and monthly 25% water changes. Water always clear, no odor, and tested fine per cheap walmart Jungle test strips.
Fast forward to August 2013 and I switch the tank over to tropicals. Before doing so, I took out the plants, did a complete gravel vacuum along with a 25% water change. Next day, I took acq water to petland who did the "liquid testing" and my water was "pristine."
I then stocked the aquarium with fish. I go to forums and aqadvisor.com only to find out I'm overstocked and I should be doing WEEKLY 30% water changes.
WHAT ?????? So I do the aqadvisor.com thing on a retro basis only to find that even without the pleco, I should have been doing 78% PER WEEK water changes with the goldfish .... yet I was doing 20-25% per month with no problems. So why did my water test perfect at PetSmart after only a 25% change ?????????? ---- So what do people think happenned in the last 15 years that made water changes more frequent with BETTER filtering mechanisms (probably three to four times the GPH flow of yesteryear's inferior undergravel filters)? [All this when similar advances in car engines and synthetic motor oil let the oil change interval increase from 3,000 to 10,000 miles.]
I have to use bottled spring water due to poor qualtiy tap water supply so changes are expensive. I plan to do biweekly 20% changes in my "overstocked" tank at most. I did just order the API Master test kit ..... but I think monthly 25% monthly changes for a non-overstocked tank should be fine. If my test results come back bad when I get my kits, I'll eat crow and I'll just suck up the extra $10-$20 a month for more frequent changes !!
Note: pics of my aquarium & list of my fish are in my profile. (39 fish now ......... added two more albino corys)