It really varies with the style and frequently the strength. It's one of those semi-contentious issues that not everybody exactly agrees on, but I suppose that's because taste is subjective.
Most hoppy beers will see their hop flavor, or punch, start to decline relatively soon, as in months. Hefeweizens are better enjoyed young to get the full benefit of the esters and phenols which will decline with age.
OTOH, really strong beers like imperial stouts and barleywines will greatly benefit from long-term aging, often around a year, and sometimes more.
So, long story short, it all depends.