For those who came in late, Google collects a vast amount of data on users' behaviour, including a lifelong record of web searches, locations, and YouTube views. Lately, it has had a privacy backlash and ongoing regulatory threats and the company has started to hype its recently released privacy tools, like the ability to automatically delete some of the data it collects about you.

However, according to David Dweck, the head of paid search at digital ad firm WPromote auto-delete tools accomplish little for users, even as they generate positive PR for Google.

He said that by the time three months rolls around, Google has already extracted nearly all the potential value from users' data, and from an advertising standpoint, data becomes practically worthless when it's more than a few months old.

Dweck said: ""Anything up to one month is extremely valuable. Anything beyond one month, we probably weren't going to target you anyway."

Dweck says that in the digital ad industry, recent activity is essential. If you start searching on Google for real estate or looking up housing values, for instance, Google might lump you into a "prospective home buyers" category for advertisers.

That information becomes instantly valuable to realtors, appraisers, and lenders for ad targeting, and it could remain valuable for a while as other companies, such as painters or appliance brands, try to follow up on your home buying. Still, it's unusual for advertisers to target users based on their activity from months earlier, Dweck says.