That depends entirely on your taste and the cigars you're aging. As a general rule, strong cigars age better because they retain flavor longer than mild cigars. At the same time, they mellow and become smoother, a process often referred to as "rounding." Another rule of thumb is that thick cigars, which tend to be more complex because they are rolled with more leaves, age better than thin cigars such as coronas and lonsdales.
But there are many exceptions to these rules. In the end, the tobacco blend is the key factor in a cigar's aging potential. For example, we once smoked two lonsdales made in the late 1970s: an H. Upmann 2000 from the Canary Islands and a Partagas 8-9-8 made in Jamaica. Both cigars had been stored in the same humidor for almost 20 years. The H. Upmann was packed with rich flavors of nuts and cedar, but the Partagas was thin and perfumy, an unappealing, common characteristic of overaged cigars.
The only way to make sure your cigars don't pass their prime in your humidor is to periodically test them. Lay a box down and smoke one at regular intervals, noting improvements. When they reach the strength and roundness you desire, they've aged long enough.
For more on aging cigars, watch the above video with Gordon Mott and David Savona.
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