saving datastores when player leaves, how effective is this?

So i want to save player data when the player is, but i'm wondering.. how effective is this?

Is there any chance the server will shut off if it's the last player to leave, and not have enough time to save? Thanks.

I have wondered the same thing, if the server shuts down, will it have enought time to save? Probably not, but your best bet to a good save system, is using pcall()

Here's a page on the Roblox Wiki that will show you how to do this http://wiki.roblox.com/index.php?title=Saving_Player_Data