Year of the Rooster,
How heavily stocked are your tanks?
I usually give the advice of weekly water changes because it is generally considered "fool proof." Note: I'm not indicating that anyone here is a fool!
I too have several tanks; all are freshwater:
4--eighty gallon tanks
2--55 gallon tanks
1--29 gallon tank
1--20 gallon tank
and several 10's
I let nitrAte creep drive my tank changing schedule. Whenever the nitrAtes get above 20ppm in any tank, I change out up to 100% of the water by draining the tank 75%, refilling it and then draining another 75% and refilling. This usually gets the nitrAtes below 5ppm. Having done this for years, I know how long it takes for each tank to get up to 20ppm without constant testing.
My 80 gallon with a single oscar gets the full tank cleaning twice a week as oscars are extremely messy fish. On the other hand, my heavily planted, lightly stocked rainbow fish (80 gallon) tank only needs about 25% water changed per month.
The only product that actually contains the correct type of live bacteria for cycling tanks is BioSpira. It is hard to get and often mishandled in shipment (needs to be refrigerated) and so it is iffy as to whether or not you'll get a viable batch.
My preferred way of instantly cycling a tank is to use seeded filter material from an already established tank--easy to do when you have several tanks. If you don't have another tank to borrow from, you're left to try to find someone with a healthy tank that will share some media with you.