I'd give Johnson 3 years at full max (somewhere around 50 million), if he wants 5 years I'd go with something similar to what he has now i.e. 5 years 70 million. I might over pay a bit more if I'm the Knicks or some team like that that has been bad for a long time.
And it is much better to spend the money on Johnson then let it ride to 2011 as that class is much much weaker with Dirk and Pierce being the two best players and they are both well on the downside. Other "marquee" free agents are Tony Parker, Caron Butler, Josh Howard, Kendrick Perkins, and Tyson Chandler.
In other words, overpaying for Johnson is a hell of a lot better then saving the money for the next class as none of those guys is any better then Johnson.