My personal rule of thumb is one month of secondary, plus one month for each percent ABV over 4. So two months for a 5% ABV, three months for a 6%, etc.
Is it an exact science? Definitely not. I view that rule of thumb as being +/- 1 month. So you can subtract 1 month from that and the beer will be good (5% at one month, 6% at two months), but the extra month makes a BIG difference. Adding one additional month to the aging process, though it makes less of a difference, will give you additional improvement. So we have...
4% alright straight out of primary, really good at 1 month, a little better at 2 months;
5% pretty good after 1 month, really good after 2, and a little better after 3 months
6% good after 2 months, a lot better after 3 months, a little better still after 4 months;
The only thing is, as you get even higher on the ABV chart, the beer will continue to improve over a longer period. Anything over 6% ABV starts to age nicely for some time. I think my scale still holds true, for example, that a 10% beer will be good at 6 months, even better at 7 and better still at 8, but you can draw it out even longer... it will really be vastly improved still at 12-18 months or more.
And as zoebisch01 mentioned, wheats are a little different. They seem to taste better when they are a bit fresher. They still fit in my scale, but just at the lower end of it. For example, I made a 7% honey-hefe that wasn't great after a month, was decent after 2, but really hit it's stride at the 3 month mark.
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