How often should I backup iphone 6 to my computer?

How often do you change the data on it?

Depends on a couple of things, such as how much actual data you have, and how often it changes, as well as your tolerance for losing some of it. If you are using iCloud back ups (as opposed to backing up to the computer through iTunes) you can set it to back up anytime you have the phone plugged in, connected to WiFi, and the screen locked - so pretty much anytime you charge the phone. For iTunes back ups, I would say at least once a week, more often it your have the time and would feel more comfortable doing that, or if your data changes frequently