Stamkos had 40 goals and 81 points last year, roughly the same age as JT, and was able to convert that to $8M/4 yrs. And we can be certain John & his agent are well aware of that.
Tavares is on pace for similar numbers, but even if he drops off a bit and ends up at something like 35g/75 pts my guess is that he's rightly going to be looking at $6M+ x 4.
For the Leafs, I'd recommend sticking firm to 3 years max term at something in the 6 range and hope he really, really doesn't want to move his family. If he insists on getting paid Stamkos money and/or more term than the team is comfortable with they let him walk.
Unfortunately, this is MLSE and we all know what they're going to do. If Tavares keeps this level of play up, unless the team flops again in April and Shanny is convinced JT has to be the "cap scapegoat", JT is going to get $7+/4 yrs and retire a Leaf.