long time, well longer than I'd like them to. My rainbows and my vipers all sit underwater for extended periods. My trees only soak every so often, but they'll surprise you with their lung capacity. I'll go with a guess of 20 minutes, and that's fairly conservative I'd say, rainbow boas have been timed at staying under about 40 minutes in captivity.

She doesn't need to be smart, her instincts won't allow her to kill herself unless she's totally whacked out, or has no energy, but it's usually nothing to worry about.