I find that in 30 mins a good pro can find 1 or 2 things, fix them and keep you on track hitting balls for those things. You can then keep those things up in practice. I find with an hour, even with the same pro, that I end up changing more and feeling great at the end of the lesson, but struggle to keep on track during practice with everything I've been told.
Give me half an hour, 2 weeks to nail what I've been told and then half an hour again, every time.
Edit to add: This is clearly for a 'single issue' type thing though. An hour for a 'back to basics' MOT type would seem fair. Basics with a 7iron, hit a few. Grab a driver, look at that again. hit a few. bit of short game. It depends on what you're looking for. Talk to your pro and see what they suggest.
