Anything higher than about a millimetre!'
This is because you need to know a couple of things, mainly the compressibility of the surface that the crate is being dropped on to. If you consider a concrete floor to be totally rigid, then the deceleration will be instantaneous.
For 5 G's though, you need the crate so slow down at the rate of 49 m/sec/sec. So, if the crate is being dropped at 49 metres per second, then it'll have to stop in one second to make 5 G's.
(might be wrong on that though.)
One of the only physics formula's I remember from school, also about 20 years ago, is -
S = UT + 1/2 A T^2.
S = distance
UT = initial speed & distance
1/2 A = 4.9 m/s/s
T^2 = time squared.
Hope it helps.