I would say that a fresh graduate should work for 5 years in a good company where he is exposed to various aspects of business. The friends and network created will come in handy later on also. Secondly there is scope to build some capital in the mean time. More than 5 years will be a waste as you will be giving to the employer the best years of your life.
One should also, if possible; try and get into a company who is into a similar line of busniness as one is planning to start later on. The hands on knowledge will come in really handy.