For a conservative lip lift in a male patient, how many mm (roughly) might you remove?

The important measurement isn't how much skin you remove, but how much you leave behind. For a subtle improvement that keeps you looking natural and masculine, I usually leave behind 15-20mm in men, but it really depends on the size of your face overall and the proportions of your other features. For me, male lip lifts are always a task in conservatism. Many attractive, masculine-looking men have upper lips that are "too long" (take a look at Daniel Craig), so you are absolutely correct that the biggest risk aesthetically is removing too much skin and feminizing your appearance.