Even with everyones best intentions the flow timeline or upgrade is always subject to change. Unless contractually it is stated "5 years etc..." there is no way to guarantee when you will actually be at the mainline. In 1991 at Eagle we were all told in class that upgrades were within 1 year. It never happened. A healthy buffer is always a good idea. Better yet, pick a place where you will be happy in the long run and the flow when it comes will be the icing on the cake!