I always like to keep several kegs of beer on hand but never really know when I will tap the next one. It depends on when the current keg is empty. Rather than dry hopping as soon as the beer is moved into the secondary could I instead work backwards? Could I wait until I only have a gallon or so of the current keg left and begin the dry hop then so I get the most benefit of the dry hop?
Figure out how long it takes you to get the keg carbed, then add 7-10 days (for dryhopping) to that to get a total time for dry-hop to glass. Then multiply this total by your average daily consumption (in gallons) to arrive at an estimate of the gallons you need in the current keg to keep you drinking until the next one is on tap. So if you chill and carb in four days and you drink two pints a day, you’ll need (4 + 10)x(0.25) = 3.5 gallons in the current keg to cover the dryhopping process.
I wasn’t going to get quite that technical with the math, I was going to just swag it. What I want to know is if there is any problem with having the beer in the secondary or keg for a month and then dry hopping rather than dry hopping as soon as the beer hits the secondary.
Yes to the OP.
I dry hop my beers in relation to the date I will tap them, not in relation to the date that I brewed them.
Better beer comes from getting at least a little technical. 
I want my hop-forward beers to hit the glass with the best possible hop character, so doing just a little math is a small price to pay. But yes, you should wait to dryhop until you’re close to ready to drink it.
