It took about 2 years. April 1998, Larry Page and Sergey Brin [0].

"Currently, the predominant business model for commercial search engines is advertising. We expect that advertising funded search engines will be inherently biased towards the advertisers and away from the needs of the consumers... we believe the issue of advertising causes enough mixed incentives that it is crucial to have a competitive search engine that is transparent and in the academic realm."

October 2000: Google adds ads.

[0] https://research.google/pubs/the-anatomy-of-a-large-scale-hy...