Tell him 5 years, or he can play on the tag for two years, then if he doesn't like the deal, he can walk.
The man's good, but Jerry ain't getting any younger, if Dak doesn't at least get to the NFC championship game this year, or the next, with this team, then like Garrett, he's had his chances, time to move on. Realistically in two years there will be so many good core players that simply can't all be signed, the team's going downhill anyway.
Take your best shot for two years, if it doesn't work out, start over. Go back to 1989...