I've seen a number of courses in universities where students are expected to get up to speed on Hadoop in about 2-4 weeks. My memory is a bit vague on this one, but I do remember somewhere that a mid-term homework assignment was to implement PageRank over Wikipedia articles using Hadoop. I would certainly consider that a "comfortable" level.
Of course, your learning curve will vary depending on your background and available resources. The courses I referred to above almost always require "distributed systems" as a prerequisite. The classes also usually have a
test cluster already set up. If you're setting one up yourself, factor in some time on learning systems administration.
