It's a good rough guestimate, but always remember you are dealing with living micro organisms, they have their own agendas...Your best bet is to plan longer, and not be worried if it's not ready...I leave my beers in primary for a month and bottle, and
hope they will be drinkable after 3 weeks, but usually it takes a little longer, especially if, like now, my Loft is only in the 60's I haven't had any beers carbed in three weeks...
And I ain't worried.
