Single digit percent of Google's overall ad inventory
would be $1bn-$10bn per annum, at current scale. Google's ad business is big. Over 10 years...
One of the first "tricks" you learn on adowrds is to turn off (it's hard to find) "search partners," or at least split it off and quarter your bids. It's on by default and'll take up a surprising amount of your budget, using default settings.
I wouldn't be surprised if it ads up to a meaningful share of the "search ads" market.