What length do I cut directly into a 1/2 inch copper tubing to install a compression shut off valve.

It depends on the length of the valve and how much play you have in the pipes. The tubing needs to seat at the bottom of the fitting.

To install a compression shutoff you will need a minimum of 1 inch of copper exposed. If you plan to install a chrome cover behind the shutoff you will need a minimum of 1.25" exposed copper pipe.

MARK