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The period without ads lasted ~4 years, during which almost nobody used the web, and even fewer used a graphical browser.
Mosaic 0.5 was first released in January '93 (yes, there were other graphical browsers preceding it, like Viola, but none saw broad distribution)
Netscape was first launched in October '94.
By '94 the first banner ads existed.
DoubleClick started selling banner ads in '95.
Ads were certainly very rare in 1993.