I dont think anyone will know how tesla specifically handles this for MODEL 3 and Ys which can not currently be purchased at lease end. Leasing was not offered when the car was launched in late 2017, nor any time in 2018 when I purchased mine.
I think leasing didnt become available for model 3s till some time in 2019... a quick internet search looks like April or so of 2019. There might be some people who got 24 month leases who might be able to answer the question, but the "standard" lease term in the US tends to be 36 months, as much as something like this can be standardized.
You likely are looking at april of next year before a lot of people see how tesla will handle this for model 3s. I suspect (but dont know) that if you have another tesla on order, you will be able to extend the lease as mentioned, as that is pretty common.
Will need to wait till someone gets to that point and is also a member here, and also decides to post about it, before we know.