This really depends on the size of your tank. I used to keep a betta in a really small tank with no plants or ornamentation. Waste builds up quickly, so I had to change the water very often, meaning half of it every other day. Here’s how my tanks have evolved since then:
I moved to a five gallon tank with two bettas, a divider, and a small filter. Overall, the water was clearer, but I still needed to change at least 1/4 of the water every week. I also had a five gallon with one betta, two otocinclus, and a dwarf frog. The extra critters made for minimal waste, but I still replaced 1/4 of the water every week, plus a new filter cartridge every month.
Right now, I have two tanks, one large and one small. My large one is a 20 gallon low, with one betta, five neon tetras, two otocinclus, two dwarf frogs, and one siamese algae eater. There are many plants in there. This tank has a filter rated to 30 gallons, and I change the filter once a month and the water by 25% every ten days or twice every week, depending on water tests.
The other tank (my daughter’s) is a five gallon containing three glo-tetras, and two fake plants. I change at least 25% of their water every week.
For both tanks, I use a siphon to vacuum the substrate while extracting the water.
Keeping your water clear depends on the size of your tank, but population and filtration also come into play. If you have a small tank (especially with no filtration), change the water often. You have a little more play once you introduce filters, plants, and capacity, but keep your fish happy with a suitable amount of new water regularly.