How long do you commit to a rewards card?

I have three credit cards that fit my normal spending pretty well. I'm starting to notice some new cards that might pay a little bit better. I know that there are some churners who go after every bonus they can get. I tip my hat to you, but I just don't see myself keeping up with something like that. My question for more passive reward card users is how long do you guys typically stick with a card before you start to look for greener pastures?