About 500-600 AGL is the demonstrated, proven altitude from flight testing. It depends on the model year. Newer aircraft (I think about 2014 or 2015) need a slightly higher altitude, closer to 600 AGL, because they have a higher max gross weight and correspondingly larger parachute system.Never flown a Cirrus. What is the absolute lowest AGL altitude you can be to pull the chute, as in have it deployed fully, and right then make contact with the ground?
The altitude loss also varies based on aircraft attitude and airspeed. Upright, wings level, with reasonable forward airspeed, as you probably know, is optimal. The system will work in a wide spectrum of conditions, but requires more altitude if the aircraft is spinning, inverted, or in a steep dive.
All of this being said, there have been successful deployments as low as a couple hundred feet. Although I chalk those up more to good luck than anything else. The system isn't designed or tested to work that low.