What are you using? I have Protanks mainly but use the coils in my other clearos, too. If it tastes burnt, yep, time to change it. If the flavor isn't anything like I'm used to, I either rinse them off or swap it out. If I'm not in a hurry, I'll do a dry burn and change only the wick or rebuild one.
If you're using a replaceable coil head, don't throw them away when you change them. You never know, you might want to build one eventually.