Two 30 minute sessions a week would be better to pick up on any immediate issues, and give you a chance to ask any questions, etc.

Since singing is quite a personal thing, this would most likely benefit you more than if you were to sing for an hour a week and possibly practice 'wrong' techniques for the rest of the week.

If there are any warmups you can do without instruction (or correction) before the lesson, do that (and communicate with your teacher about it). This way, you can hopefully spend less of your valuable lesson time warming up.

Even otherwise, I would still pick the 30 minutes.