You should withdraw money when you feel the bitcoin is showing signs of market bubbles. Because sometimes you can see the bitcoin is pushed too fast and the market can not accept it. That's when you need to sell the bitcoin, but then buy a bitcoin at a lower price because you know where the future of bitcoin is going.
I think we cannot call bitcoin market as bubble anymore as when ever there is a surge it sets a new benchmark and the value does not go below that so it will not be fair to call it as bubble rather cashing out the funds purely depends on the needs of the holder as if it is some sort of emergency or need financial help or going through financial crisis then I would suggest to withdraw the funds as the sole purpose of holding bitcoin is to have sufficient asset when you need it so there is No harm in cashing out for needs.