The Tarrasque sleeps for 6d4 months at a time, and then wakes up for 1d3 days to eat. It's not hostile, just hungry. If you're flying, have a decent head start on it (which, considering that the area around T's lair is more than likely going to be relatively flat, isn't too improbable-how hard is it to spot something the size of a small castle?), and/or are lucky, there's a decent chance you'll be able to escape alive. Or you could just find the area where people and towns keep disappearing with no trace, and a few divinations/legend lores should let you figure it out.