Trace Tesla's history, and you will see that most models average a 4-6% increase per year. That's actually a pretty big range, but it varies by vehicle and trim. The Model Y recently say an increase of less than 5% to get to 326 miles on the longest range trim. Just going on past trends, it seems likely that it will get to 350+ somewhere around 2022 or 2023.
Obviously this is all guesswork, but its been pretty consistent for quite a while now.