I'm straddling the middle two options honestly. My campaigns have almost always run from 6-18 months. I've had the odd one go 2 years, but, that's pretty rare.
In answer to Lanefan's question, to be honest, I've never had a GROUP last more than a couple of years until very recently. Most of the groups I belonged to split up after 6-18 months due to school, drama issues, or moving. I've personally moved an awful lot over the years, so, the idea of trying to keep a five year campaign alive was just completely unfeasable.
Honestly, Until I got well into my thirties, I've never been in a stable enough situation, nor have anyone I've gamed with, to ever presume that a campaign would last so long.