Right now I do kettlebells mostly every other day, although sometimes I'll go back-to-back if I'm doing more cardio-based kettlebell work with a lighter bell and I feel good.
If you're combining kettlebells and kickboxing, my only suggestion is to listen to your shoulders. Both kettlebells and kickboxing (more specifically, all the punching) involve a lot of shoulder work - these are two of my favorite types of workouts, so I've done kb/kb rotations before, and I remember some sore lats and shoulders.
You don't want to mess around when it comes to your shoulders. Rotator cuff surgery is six months to a year of NO FUN. So definitely include a rest day, and if your shoulders are sore, do something non-shouldery to give'em a rest.