There is a study on pubmed (sorry can't find link right now) where participants used rogaine 5% 2/day for 1 year and then half of them switched to 1/day for the subsequent year while the rest stayed on 2/day. At the end of the second year, the group that switched to 1/day did have lower hair counts than the group that stayed on 2/day, but it wasn't a huge difference.
