I'm on about the 5th beer aging in these barrels, trust me, it can get over oaked very quickly. The best way to go about it is taste. But 4 weeks will be pushing it. Personally, I find most commercial breweries over age their beers in barrels as well. I pull mind a lot sooner than others. Sometimes, even in a 60 gallon bbl you can pull it after 4-6 weeks.
I'm not saying it won't taste overoaked after about 4 weeks. I'm saying it comes through that and into something better after a few more weeks. There was a presentation at NHC 2013 about alternative wood ageing and the presenter went into the specifics of it.
That doesn't match my experience at all. One of the major problems you are overlooking is also oxidation. beer that has sat in bbls too long turns into an oxidated mess. I have a small barrel program at Yellowhammer and in my experience it is very critical to taste and pull the beer at the right time or you will ruin it.
If anyone get's a copy of "New Brewer" look at the barrel aging article by the guy over their program at Avery brewing. It's a very good article and echoes what I'm saying. Even in 60 gallon barrels I find that often I have enough wood balance after 4-6 weeks.