I presently have 7 tanks running, including a 115g, 90g and 70g. I do the water changes in one day, it takes about 3 hours, and currently i do them on Monday in the morning. I was dividing them between two days, half and half, but after a few weeks I decided I preferred doing them together. The only downside to this is if i want to do some re-aquascaping in one tank; so this is sometimes best left for another day.
One reason i went back to one day was keeping better track of plant fertilization which is twice weekly. It is easier for me to remember doing it every Tuesday and Friday morning, rather than Tuesday and Friday for these tanks, and Thursday and Sunday for those tanks... or whatever.