60 vs 30
I take 60 minute lessons, and feel like if they were any shorter, i would get absolutely nothing done. After 60 minutes sometimes i feel like i want more, just to fine tune what i just learned.
As far as how regularly you take lessons, i believe is totally up to you and what you feel you need, and how serious you want to get in a certain time span. I take lessons usually every 2 weeks. sometimes 3 weeks, but never more than that. on occasion i will take a lesson on back to back weekends, if i have a tournament or something coming up, and issues with a certain aspect of my game. 2 week intervals seem to be perfect for what i am striving for, however i believe it changes person to person.
I think i will never finish taking lessons, i am sure the regularity of my lessons will dwindle, but i doubt i will ever stop taking lessons for life. I think a lesson every 2 months or so would be beneficial to my game... hell, even the pros have "coaches" that they see regularly, so why stop. but maybe once you reach your desired handicap and feel comfortable with what you learned you might decide that its time to call the lessons quits.