I don't know about Blizzard, but with UO and EQ, they're (damn near?) permanant. I quit UO for over 18 months, came back, and still had all my characters and shiz (sans house). EQ the same story, but it was only about 8 or 10 months.

Blizzard did get delete-happy with Diablo, though, so I don't know what kind of comparison you can make. I'd trust that they'd just leave them there, at least for the time being.

What is it they say about silence being golden?