I've been retired for 3 years.
I withdraw 3 months' worth of expenses at a time and put the proceeds in my savings account. I move about 1 month's worth of expenses from savings to checking each month and spend from my checking account. When the savings account runs low, I withdraw another 3 months' worth of expenses.
Three months is what feels like a comfortable balance to me between keeping my money invested and not having to watch my cash account balances very closely all the time.
I rebalance occasionally whenever I notice I am out of whack relative to my target AA. I consider my portfolio AA as a whole and rebalance within my traditional IRA, so rebalancing and withdrawals are not connected or related at all.
Other than the money that I'm planning on spending in the next few months, I do not keep any sort of cash cushion in my portfolio at all.
So my simple rule on when to sell is when I need the money. I realize there will be times when I will sell at relatively low market valuations when I need cash and the market is down. I think that during those times I will naturally become a little more frugal. I also think that the drawback of having a cash cushion all the time outweighs the benefits that only occur some of the time. Also, a cash cushion needs to be refilled, and the complications associated with that add, well, complexity.
My target AA is 93/7 and does not depend on whether or not we are in a bull market, a bear market, or otherwise. I do not believe in trying to change one's AA in response to current or recent market conditions.