from my own personal experience, there is no hard and fast rule when it comes to churning.

each individual is assessed differently so there will always be a YMMV when it comes to giving advice.

the best advice anyone can give you is to research and read. Once you've done that, the next step is to ask whether you are comfortable with the risk/reward ratio you have discovered through your readings. if you are comfortable, proceed with your churning. Once you've done it a few times, you'll begin to understand the process that is unique to your situation.