I am a certified licensed HVAC technician and a auto A/C licensed technician for more than 25 years. To answer your question, it depends on the auto say you had a all car it should take no more than 16 to 48 oz. But a large SUV or a van they usually have 2 units one for the front and one in the back. Now they could take as much as 4,5,6, pound's or more. Also there is a low valocity oil mixed in with the freon most cars take about 8 to 10 oz so if you loose all or some of your freon you need to put some oil back in also , and one more thing, you said the A/C units in these cars has not worked in some time, well you will probably have a 50 50 chance of the A/C units that will blow out the compressor seal's and all the o rings cracked and dryer out because like I said there is oil in the system with the freon and if your A/C units have not been used in a while to keep all the rubber that includes the rubber hose line it will dry out and when you start charging a system with freon the pressure could crack open the seal's in compressor, o rings and rubber hose line will leak out all your freon. And if that happens to the seal's in compressor there is nothing you can do except kiss it goodbye. A/C system,s must run from time to time to keep the system lubricated