Adblock Plus should start working instantly. (I'm assuming that you downloaded it via Tools > Add-ons > Extensions and 'Search all add-ons', and not from somewhere else).
If you're getting ads on the right-hand-side of this page in the column below 'Latest Posts' then Adblock Plus isn't working for some reason. Take a look under Toiols > Add-ons > Extensions to see if it's listed there. If there's an 'Enable' option, use it. Otherwise try uninstalling it and then reinstalling it.
If you've got no ads down the right-hand-side of this page, but you keep on getting advertising pop-ups, then Adblock Plus is doing what it's meant to do (i.e. blocking ads embedded in web pages). However you've got malware on your computer (producing those pop-ups) which needs a separate solution to deal with it. Download, install and run the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/antimalware/
(NB: When installing it, take great care to deselect the pre-ticked option to take a free trial of the Premium version).