Not being passionate about what you're doing at work is your sign that perhaps it's a good time to leave. Of course, find another job first and then leave.

Before you do that, take an honest assessment of why you're no longer passionate about your current employer. You need to understand why they no longer scratch your itch, why they no longer excite you. See if you can identify traits to be on the lookout for when seeking new employment. You'd really hate to leave one place that's not exciting you just to go somewhere else that's not exciting you, either.