After reading hundreds of threads about Proofhide and Brooks saddles, I have the answer: it doesn't matter.
Do it once, or do it once a month; you won't notice the difference.
IMHO the best thing for a Brooks saddle is riding it. If you put it into storage and/or don't ride it for awhile, it will get a bit dried out. So, put something on it when you get it out of storage. Then ride it.