How often do you change your Ultimate Team? ?

I tend to get bored with my team after a week or two and start looking at different players. The problem is that changing the squad too often can use up a lot of coins, especially if I keep buying and selling cards. At the same time, trying new combinations is one of the most enjoyable parts of Ultimate Team for me. Do you stick with one squad for a long time, or do you regularly rebuild it? How do you decide when a player is actually worth replacing?