You might have been feeding the bit a little too slow. Usually when I burn something that's why. How much life left is impossible to answer. Depends on the woods you rout and technique.

If you are routing pitchy wood clean the bit regularly. That can help.

 

Yep. I was slow feeding into red oak. So is there any good way to check if the bit is still in decent shape?

 

The quickest way is to use it again. I doubt very much that the bit is finished. That is why carbide is so popular, it can take lots of heat. Pass it through a piece of scrap and check the finish and how it felt as it went through. There is no way to check it other than microscopic inspection. If you don't trust it, throw it away and get another one.

Otto

 

I realize I didn't read your original post closely enough. Make sure the bit you are using is meant to be plunged. Not all straight bits are. Unless you are plunging a small distance I would drill a starter hole at the least and possibly hog as much waste out by drilling as possible. Most drill bits will remove waste as fast as a router and much more cheaply.

 

Being a machinist I've seen carbide with steel chips rolling off of it red hot and you'd be amazed at how long they'll last doing that. Like Cherryville Chuck said if your bit is designed for plunge cuts you didn't hurt the carbide. Make sure to clean any pitch off and it's likely not hurt at all.

 

Until it gets to dull to cut cleanly.