The old rule of thumb is that you should have your age as a percentage in bonds, the rest in stocks (eg 30 years old = 30% bonds, 70% stocks). Since life expectancy is increasing some people suggest modifications to that, eg age minus 20 in bonds
But if I were in my 20s I would hold a few months' living expenses in cash for emergencies and put 100% of the rest into stocks.
The Investor's Manifesto is an excellent book on this