Not sure, but I guess you will get a prompt to begin the installation or decline. I reckon it will keep prompting you periodically to begin an installation which you can decline each time (hopefully!)! Note that from July next year Microsoft may not be giving away a free license, so you should have at least that long to decide.
See some recent info here:-
http://www.theinquirer.net/inquirer/news/2425381/microsoft-is-downloading-windows-10-to-your-machine-just-in-case
As for windows 10, you can install it and if you don't like it you have an option to uninstall and return to your previous system, but you must do that within a 30 day period, else you wont be able to return. Have carried out the uninstall process twice now and it has worked ok for me.
Having said that I have it running successfully on 3 Laptops and have experienced only one really annoying problem to do with the bundled Microsoft Anti-virus package 'Windows Defender'. After the first month or so it suddenly decide to stop updating its Anti-virus database on one of the laptops and no amount of tweaking or complaining to Microsoft seems to have sorted the problem. Still works ok on the other two laptops.
Probably have to reluctantly replace Windows Defender with another free anti-virus product and in the meantime hope that either myself or Microsoft sorts it out in the background (we live in hope!).