Rapid changes in temperature, hardness or pH stress fish and are often the cause of outbreaks of diseases like white spot. I have a set up where I blend hot and cold to about the same in the tank and can then often do very big water changes. Temperature is however only one of the factors it is best not to change too rapidly so the size of the water change usually equates to the frequency. If I have been a bit slack with water changes I usually do a number of smaller changes to catch up.