I did my grade 8 singing last december after starting lessons with my teacher in september and having done no other singing exams. I managed on once a week of 30 mins, so it's entirely possible.
When I started working on my DipABRSM, my teacher asked me if I would like to go up to hour because she thought it would be more beneficial. She didn't force me too, but I took the offer.
I also took grade 7 clarinet with weekly 15 minute lessons from the school peripatetic woodwind teacher. It just means that you have to do more work at home!
A 45 minute compromise sounds like a great idea.
