Either two to three months after a new model intros, to allow time for necessary firmware patches, system updates etc.; or else during a back-to-school promotion if you're looking to bundle it with a discounted printer or iPod.
Subject, of course, to you actually needing a new one.
The most recent update was a particularly big one:
A vastly improved wireless card that uses much less power
Integrated graphics built into the CPU, again saving power
A big increase in VRAM for most models
Bigger hard drive options on most models
Thunderbolt (FW800 was previously the fastest I/O on most models)
SDXC slot (was previously only SDHC)
Quad-core on 15 and 17 inch models
Lower wattage (25W maximum) processor on entry-level 13 inch model
New features on the CPUs that considerably boost speed
Faster discrete graphics on 15 and 17 inch models
About the only thing they didn't improve aside from screen resolution was the 3D graphics speed on the 13-inch models; that was a mixture of slightly better to slightly slower depending on the application. But even the 13 sees big improvements on CPU-intensive photo applications like iPhoto and Aperture.
The big thing on the next generation model is likely to be a big reduction in power consumption due to the "3D" 22 nanometer chipset. Possibly also significant further speed improvements and changes to the case design, though we'll see.