I guess it depends on the horse and the situation. I don't buy alot, bought more over the past year then I have in probably 10 years, not for me though. When I bought my mare I made an offer the first visit, had to come back a few days later to get her though (didn't bring a trailer). Also bought a pony, and only needed one visit, but I already knew she'd be a project.
Bought my sister's horse on one visit, but he was to far away to leave and come back.
Don't know if she counts, but I also helped the neighbor's daughter to find a horse. We visited her 3 times, BUT the first time we weren't actually there to see her, we were buying the pony. When the neighbor asked me to help her sale her current horse and find one more suitable for her, we went without them to try this mare out (their horse hadn't sold yet), and brought them back after their other horse sold.
For my dad's we visited twice, met him the first time, and came back the next day and tried him out w/ the trailer.