Tom,
Sorry, I've been catching up with email post-vacation.
I'd say we initiated the actual migration in earnest about 10 months ago. We had tested some single item collections before that period, but we delayed the big migration until we had some sort of logic for compound objects figured out. CONTENTdm makes compound objects very easy to create, so deciding how to port our myriad compounds took some time. Additionally, due to an unfortunate server error that wiped a portion of our collections ready for ingest and index, a couple of those months were spent repeating work. Had that not been the case, I'd say about 8 months have been spent in migration.
Also, as we noted, Heather and I were not able to devote 100% of our time to the project. It accounted for a major portion, of course, but we each had our respective day-to-day duties and other ongoing projects in the library. If I come up with a more concrete number I'll certainly add it.
Tyler