Well, it lasted a little longer than normal during today's Galaxy session, I think. Normally it's less than an hour.
Interestingly, it gets quite hot, could this be causing it? My laptop gets much hotter than that when playing games like Oblivion, so I didn't think much of it. But maybe the Wii is very fickle about heat? I know Nintendo doesn't tend to use as powerful cooling systems as other consoles.