Whats the longest people have kept seahorses.

[rayjay]

Alas, he has since gotten out of the hobby/business.

That's understandable as in my ten years in the hobby I've seen people come and go so often. It's not a hobby for everyone as seahorse keeping is more demanding.
Then too, if you are raising fry, it is a WAY more demanding (and often frustrating), especially if the fry are pelagic like reidi or kuda rather than benthic like erectus.
As for lifespan, seahorses kept under proper conditions have no problem with 5 yr lifespan, and often up to even ten years.
Dwarf lifespan though is considerably shorter at 1 1/2 to 2 yrs.