Never. You can set your browser to better manage the bullshit you don’t want. Obviously some sites are designed to oppose these measures and/or you allow it in order to use the site.
For the most part I get no adverts (even blocks most YT pre ads) and they are about random things, not stuff I’ve previously searched for. So it seems to work for me for now.
Don’t log in to google every time you go on the internet. If you use IE avoid toolbars and add-ons other than Flash and Java which can be set to be slightly more robust. Manage cookie usage deleting after and/or during a session, don’t allow tracking requests, use built-in pop up and ad blockers from legitimate sources. Ideally use your browsers secure option on sites you deem to be taking the piss or avoid them all together (if possible) and find alternative sources.
You can go much further if you like!