Back when I was in high school, I built a bike just for wheelies.

Stingray frame, bannan seat, heavy duty (read heavy) BMX rear rim, smallest chinring i could get on the front (probably a 30) and the largest cog i could get on the back (probably an 18, but sure it was under 20)

Max speed on the thing was MATBE 5 MPH, but I could wheelie forever. Up hill, down hill, around corners.
I rode the two miles to school on one wheel, rode around the halls after school on one wheel (after the teachers had gone).

Can't say the longest, but it was as long as I wanted, once you learn the baance point, it's all easy. A little pedal, a little brake, a little lean, go anywhere you want.