Lat blends tend to start going flat around 8 to 10 years. Aged Virginias are a thing that few would part with. Even after six months in a jar, the change is amazing. I have blends like Red Cake, and Christmas Cheer with over a decade on it, that no amount of money could part me with.
Flat, in my experience, is a bit of an over statement. I be smoked a number of English blends with 30 years or more age and they were some of the best smokes I can recall. The biggest problem I’ve found with these older English’s is that, once opened, the blend does tend to peter out in a week or less. At least when the tin is relied upon for storage of the open blend.
Lats fade over time. How rapidly they fade depends on the conditions under which they're stored, so there's no hard and fast rule regarding this.
In a general way, Lats lose a little bit in the first 10-15 years, and by 30 years they're often just a ghost. But with some of the older tins, like cutter tops, the seal is significantly better than in newer flimsier tins, so you occasionally find a really old tin where the Latakia has mellowed, but not disappeared, and these do provide a sensational smoke, in part because the other Oriental tobaccos have matured and ripened.