Long term an 80% bond allocation will kill your portfolio quickly. If you are trying to time a major drop and then switch to 80% stocks it will be very hard to determine where the bottom is, or where the top is, so you might as well get into more stocks. If you're risk averse, I would suggest 60% stocks and 40% bonds. If you don't mind some risk I would suggest a range of 75/25 to 85/15. The heavier the stock balance, the longer it will last based on historical returns.
You need to keep enough in bonds/cash to fund a few years of expenses when you are in retirement, or as you approach retirement. Anything above that will likely drag down the performance level of your portfolio in the long run.