Atmospheric pressure at 30 000' : ~ 4psi.
Assume adiabatic expansion:
Pressure x (Volume ^ 1.4) = constant. [1.4 is a typical value for a diatomic gas, like oxygen]
Therefore:
Pressure1 x (Volume1 ^ 1.4) = Pressure2 x (Volume2 ^ 1.4)
(Volume2 / Volume1) ^ 1.4 = (Pressure1 / Pressure2)
(Volume2 / Volume1) = (Pressure1 / Pressure2) ^ (1/1.4)
P1 = 1800 psi
P2 = 4 psi
Therefore:
(Volume2 / Volume1) = (1800 / 4) ^ (1/1.4)
(Volume2 / Volume1) = 78.6
That is, the gas in the cylinder will expand to almost 80 times its compressed volume if the cylinder ruptures.
That assumes that the cylinder didn't lose any heat while it climbs to altitude - fairly reasonable, I think.
If I've messed up the maths, someone please let me know.