Speculating on Apple release dates in nearly impossible.
Check out the MacRumors Buyers Guide:
https://buyersguide.macrumors.com/
In general, its been a relatively short period between the last powerbook update.
We know it won't be the G5.
So, that leaves G4 speeds. Currently, I think Moto will have a hard time bumping more than 100 mhz if that. Freescale is too far away to be useful to us before spring of next year. So, when the next updates occur, logic dictates we will see an small increase in MHZ, and a bump in video chip set to perhaps a 9800 mobility.
So, if you need a laptop now, buy it. The new ones will not be leaps and bounds better for some time.