If earth's magnetosphere and atmosphere suddenly disappeared, how long would it take for the oceans to boil away?
It's hard to picture earths atmosphere suddenly disappearing while the effects of gravity are still active .. the previous answer is correct that new atmosphere would be formed although if gravitational effects were negated then yeah it would be gone real fast . The earth is constantly bleeding atmosphere due to solar winds but we have plenty to spare .
The magentosphere protects us from this solar wind so if that were to weaken like it did on mars the rate of loss would increase dramatically and be lost over the course of several millennia .
So bottom line if you want the oceans to boil off immediately gravity is your main obstacle there .