Roughly when will the 8th-gen consoles, if any, be released?

I thinking of buying a PSP-3000 and PS3 Slim this year. If the 8th-gen consoles are gonna be out around 2015, then I'll buy 'em. But if they're released this year or 2011, then I'll save up a bit more for the newer ones or just stick to Windows. When couldl the 7th-gen consoles become obsolete and roughly how long is a 'generation' of consoles expected to last?