how long will Butane last in a closed loop extractor

I am looking into investing into a small closed loop extractor to make BHO.

can anybody with experience please answer some questions

1. How much BHO product will 1 can of butane produce

or

2. How long until all the butane is gone in a closed loop?

or

3. when should I replace the butane?

If you have a tain collection unit you can recollect 99% of your tain after each run. You never need to replace the tain.

End product is heavily dependent on starting product.

With a passive system, one volume of vapor is typically lost at the end when you open the unit close to atmospheric pressure.

With a powered system recovered to -22" Hg, about 25% of it's internal volume in vapor is lost per cycle, when the chamber opened.

Vapor to liquid ratio is around 235:1.

Thank you for your replies. It was very helpful.