I think it depends on how good of a snowboarder and teacher you are as well as your level of patience. I've taught two girls how to snowboard over the last two years and it is a bit of work but can be difficult at times, but was well worth it an both were linking turns within a few hours. I would suggest a lesson unless your an advanced level snowboarder and can ride switch very well. Here's how I did it:
1. Explain the basics before getting on the lift. Pedaling, linking turns doen the fall line, and the fact that you always want to be on an edge.
2. Sit on the lift facing her so you can hold her hand getting off the first couple times, catch her, or help her up.
3. I started on an easy green. Too little slope is more difficult.
4. Teach her how to ride her heel edge and stop by holding her hands and riding on your toes downhill of her. Beginners first biggest fear is not being able to stop. Once they catch an edge this changes
5. Teach her how to switch to her toe edge by holding your hands and using you as a pivot point. Explain that you can stop on you toes just like your heels.
6. Teach her how to switch to heel edge by opening her shoulders and gradually shifting her weight to her heels.
Repeat these steps untill she is comfortable. Once she is, teach her the falling leaf (back and forth side slip on heel edge) for situations where it gets too steep for her to link turns.
I took a 3 day trip with a girl I was seeing last Winter (still great friends) and by the end of the trip I brought her down a few blacks at Steamboat. Granted not the most difficult being Steamboat, but still, I was impressed, she was wayyyy better than I was on day 3. She's so stoked on snowboarding that she's moving to Summit County for the winter and I have an open invitiation. Second peron I've taught to snowboard and has enjoyed it so much they moved to CO.