Honestly, I think that expecting much more than a 5 or 6 years before each company releases a follow up system when there are three companies producing consoles is unrealistic. In a two console market (like the current handheld market) it is possible because as long as the leading console is making a crap ton of money and the trailing console is well supported (and has a growing userbase) there is little reason to release a new system; in a three console market one of the companies will feel the need to get a jump start on the other two companies in order to take advantage of newer technology in an attempt to controll the "next generation" console market.
Consider that in 2010 we will be be using a 32nm process (or possibly 22nm process) which will make a fairly inexpensive GPU more powerful than the current three card Geforce 8800GTX SLi setups, a farily inexpensive CPU much more powerful than the bleeding edge processors of today, and you will be able to have 8 to 16 times as much ram. The engine developers will all have cutting edge game engines ready to take advantage of these components, and there will be hundreds of PC developers who are looking to port their games to a console which can handle them in order to limit their risk.
One of the three will want to take advantage of these conditions, and the other two know that if they're given more than a 12 month lead they will (likely) gain such a large lead that they can never be caught.
