The valve in the cylinder opens & closes pretty reliably around 1/2 a turn from fully seated on both the pistol and the fill adapter.
It takes between 3/4 of a turn to a full turn more to bleed the compressed gas out of the fittings. That's the period where the threads are under pressure, but the cylinder valve is closed.
On the pistol, the cylinders disengage completely after about 3 3/4 turns total. The fill adapter takes about an extra half turn for some reason.
If you vent the adapter fittings after unscrewing the cylinder 3/4 of a turn (just to be certain), you can save a little wear on the threads. From what Scott said, it's apparently not a practical issue with the Steyrs.
There are other pistols where it IS more of an issue. The Hammerli LP40's use a fine thread, and with aluminum on aluminum, they are prone to galling. The O-ring seal doesn't vent until you've mostly unscrewed the cylinder. Because the valve in Benelli Kites and Morinis are opened by pressure, you can vent the fill fittings before you begin unscrewing the cylinders. By the way, the Morini & Kite fill adapters are functionally identical.
Because the MIT team uses a range of pistols, we try to get the students to follow the best possible practices, regardless of what type of pistol they are shooting. We have a fill system from Best Fittings (mentioned earlier). It consists of:
'A' Clamp Kit for Buddy Bottles - No Hose: #04M1-KT04
http://www.bestfittings.co.uk/shopdispl ... Clamp+Kits
232 Bar Din Female - 1/8 BSP Male: #03M1-FM02
http://www.bestfittings.co.uk/shopexd.asp?id=200
NOTE! You need to specify that they ship the DIN adapter with a 1/4 BSP adapter, so that it can screw directly into the fill adapter. The standard catalog part has a 1/8 BSP thread. Make sure they also include a sealing washer to go between the DIN fitting and the fill assembly.
This gives you an easy to read pressure gauge (useful for Hammerli 480K's that don't have a gauge), and a vent screw to depressurize things before unscrewing the cylinders.
