It's a good question! But a tough one. Best ask Nielsen, they're good at this sort of thing. Between April 2007 and February 2008, the company conducted research on the daily habits of gamers in two age groups: 10-26 year-olds and those 27 and older. They worked out how long gamers in each group spent playing each of the three consoles, how many times a day they played them and even how many days of the month they made use of them. The results? Above. For the young ones, the 360 comes out on top, and for those a little older, it's the PS3. I don't know how, exactly, they came up with these stats (could they only poll people who had all three?), but it's looking like most people polled are rolling out their Wiis on weekends or for parties (only 11 days a month for the older group) while the other two are used as everyday beasts of burden.

UPDATE - Correction time! The graph above, obtained from emarketer via IGN, is WRONG. They got their 360 and PS3 all mixed up! For the 10-27 group, everything's fine, but for the 27+ group, swap the PS3 and 360's numbers around. Making the 360 the console of choice for (least in terms of time spent) both groups.

[Nielsen, via IGN]