The Big Lead did a study and found that just 8% of players make it all the way through five year contracts in the NFL.
Just over half (72) of the contracts were reported to be at least five years in length. Only six of them–assuming Justin Smith is on San Francisco’s roster at the start of next season–made it through with the team for the duration of the reported initial free agent contract. The others are Drew Brees, Charles Woodson, Reggie Hayward, Derrick Mason, and Adam Vinatieri.
Players on five year deals actually spent an average of 2.9 years with the team. For six year deals, it was 3.1 years and for seven year deals, it was 3.7 years.
When somebody says guaranteed money is more important than the raw length and compensation of a deal, just remember they aren't kidding.
This is also something to keep in mind when high performing players demand more money. Their deals aren't guaranteed. They have a limited window to make a ton of money and play a sport where career threatening injuries are a fact of life. They owe it to themselves to try and cash in when they can because their teams will drop them whenever it is convenient.