How long does it take to compile script

I am not sure to understand, but when you are on your script in THEME EDITOR you have in the top left an information that your script is modificated and not save. If you click on save you can see in a second this information go to a green check. If not, is that you get some errors listed in the bottom of the script.

Compilation should take no time at all. It just won't happen until you change anything and save the script, or until you export your theme.