You get better long term growth out of stocks. Bonds offer less volatility which is what you want when you are depending on that money.
Until I'm seriously asking myself: "Do I want to retire yet?" I wouldn't put anything in bonds at all. Once I reach that point I'd shift ~5-10% per year into bonds, until I have enough to cover at least 5 years of my retirement. Once retired I'd pull out of whatever's grown the most that year, to live on. In years where stocks outperform bonds, keep shifting more into bonds to maintain that 5 year runway. Even grow it further to 10 years. Then I'd just maintain that 10 years of bond runway. It's highly unlikely I'll ever need more than that.