Thumbs on phone make numbers tough but you get the idea. Also some new systems require 250 or lower.

If the lines are re used and cleaned out there's a good 2 or 3 hours to flush with nitrogen then evac properly. I've never seen a 2 hour swap that worked long.

Also oil in the compressor needs to be clean and dry, fittings need to be tight and a pressure test with nitrogen is best before pulling a vacuum. All takes time and adds to the cost of doing it right.

No one even discussed changing refrigerant which requires a good cleaning with a specialized chemical product.