Anyone who knows the answer to that is forbidden to share it. There are rumor sites that look at how long it's taken between new models in the past, so that may be of interest to you. How long should you wait? The most common answer to that, and the one that makes the most sense to the most people, is to buy one when you need it.
Well the rules are here we are not really allowed to speculate about upcoming products...that is more for sites such as Macrumors.
My advice is...if you need/want one now, get it. If you don't mind waiting...wait! Nobody knows when the next will come and there will always be a better model, so don't think about it too much.
The new 13" MBP's are coming out within a year from now, certainly within 5 years, like they always do.
The 13" doesn't have a video card, overheats and doesn't have a anti-glare screen option, if you want to 3D game, your going to have to buy the 2.2 Ghz 15" MBP or better.
There is no big performance difference in 13" line that you should really care about, it's just a basic consumer machine and can't do much anyway so a small performance boost isn't going to make a lick of difference.
If you interested in 3D gaming and video needs, then I would consider buying a high end MacBook Pro when they released a new one to maximize your purchase value.
But other than that, new Mac releases don't genrate as much excitement anymore.
You can consult the MacRumors page here to determine if you should buy or not.