I put their meeting at being around 1992 or 1993. But that's just my interpretation of ambiguous information.

Wilson mentions that House has lived in his apartment for 15 years, which makes me think that he's been around for at least that long.

Wilson also knew about House's patient Esther Doyle, which was 12 years prior to the episode it was mentioned in, which also leads me to think they'd known each other at least that long.