The Wii will probably have the longest life span this gen simply because its in the lead in marketshare and still selling with record numbers. The only thing that will possibly cause this to not happen is A) MS and/or Sony decides to release their next system early and Nintendo feels they need to release theirs near the same time to compete or B) Nintendo feels they need to release theirs within the projected 5 year time span/earlier than the competition to compete and rally Third Party support (something they got screwed with by XBOX360 going first this gen).
The thing that throws a wrench in everything is how all the companies this gen has made statements about wanting to have their systems go beyond the usual 'generation' life span of 5-6 years. In other words, all of them were trying to become the next PS2. But since none ofthem really became that....who knows if they're going to focus on pushing better software on their current systems...or just trying to push for a new better system in 2-3 years. Don't be surprised of rumors for a new XBOX and PlayStation circulating around Summer of this year. As for Nintendo...you know they never announce anything until they actually announce it. But like with all their systems, you know they have a hefty chunk of R&D working on multiple prototypes for possible DS and Wii sequels. As they have said in interviews.
Six upcoming games you should look into:
