I think I am missing something here. The long times to get down to 500 microns, are they on system repairs on existing system. i.e. replacing a compressor, or reversing valve. Or are you guys talking about installing a new system, and just vacuuming from the unit to the indoor coil? On new installs, I purge with nitrogen as I weld, then pressure test with nitrogen. In one side of the system and out the other, so it never takes me more then 15 minutes to pull a vacuum down through my Brute gauges that have a 3/8 vacuum hose. Valve cores are always put back in after welding up.
On repairs I just schedule the repair to leave the pump on over night, and fire everything up the next morning. I would die of boredom waiting on a pump. And I always want to make sure everything is clean. I also always make sure my vacuum pump has fresh oil in it for the over night jobs. Clean oil makes a world of difference on the performance of the pump.