i have a similar setup on my sedan and i can answer this question hopefully for you torchmedia. the speed of the rise and fall depends more on the size of the lines, but with 1/8 or 1/4 lines you should be more than able to raise up seconds before a dip or bump or driveway. not sure how much travel his setup has but mine that has only a 3.5" travel holds about 2 full lifts worth of air. but this depends on the size of the air tank and obviously if it is full or not.
there is a sensor that kicks the motor on when the PSI drops below a preset level, but i am not sure how much air (what % of a full lift) at a minimum the preset/psi level is set to. mine hold about 2" of lift at a minimum which is still enough for me to avoid smashing into things.
i also just want to state that this is an amazing deal on the air susp parts and that i don't think this price will come around again. if i didn't just get coilovers not long ago i would have this already in the mail on its way to my z4
