Another Air Question

Have a 2004 Durtch Star. It has been parked (on jacks) for a few months (in Corus Christi). We are loading it in preparation for full timing for a spell. While it has been sitting (jacks up), I notice it has been settling down to a lower height, and when started up, raises back up to normal ride height. I think that is normal.....But...how long should it stay at normal ride height once the engine is shut off ? Does that time vary depending upon outside temperature (it seems to lower quicker in cool weather). It does not take long to raise back up when the engine is started, and the air brakes seem to be working fine on the road. Do I have a air problem..., or is this normal ? Thanks for any comments.