Not much. At the student revue last year, we had filled a thermos with dry ice, closed it up, and forgot about it on stage overnight. At some point during the night, it blew off the plastic valve at the top, which was ruined, but the metal flask itself was fine. The people sleeping near the stage reported a loud bang, but nothing else.
Hypothesis: since the valve is only going to be propelled by the expanding gases for a relatively short time before it has cleared the flask opening, I don't even think it accelerates to that great a speed.
Clarification: a thermos is more or less a best-case scenario, since the valve at the top is going to be much weaker than the flask itself. A plastic bottle is worst-case, because it is going to shatter and eject plastic splinters.
Hypothesis: since the valve is only going to be propelled by the expanding gases for a relatively short time before it has cleared the flask opening, I don't even think it accelerates to that great a speed.
Clarification: a thermos is more or less a best-case scenario, since the valve at the top is going to be much weaker than the flask itself. A plastic bottle is worst-case, because it is going to shatter and eject plastic splinters.