Safe Withdrawals
Once you build enough savings to retire early, understanding how you can then live off the money without running out is an extremely important question.
There is pretty wide agreement that withdrawing 4% of your portfolio per year is about as high as you can go without exposing yourself to too much risk. There are obviously numerous factors that impact how much you can in fact withdraw. Asset allocation, market performance and inflation are a few of the big ones. The biggest of these in my view is asset allocation. The 4% withdrawal rule assumes you hold a diversified portfolio of stocks and bonds, usually in the range of 60% stocks / 40% bonds.
If you're interested in digging into the details of sustainable withdrawals, check out the following links:
http://www.fpanet.org/journal/articles/2004_Issues/jfp1004-art6.cfm?renderforprint=1