I bought keys in shops thinking they would be needed somewhere.
As for shield, ah i see. I thought that's the 3rd shield. I think the free one is too deep to get since it requires two special tools to get them while purchasing it in shop is easier.
As for white rock, I forgot to wield Master sword to destroy that one. I thought the third sword (the one who can shoot regardless of health) could destroy white rocks also.
I couldn't give rough estimate since I play this for half hour a day. Hmmm.... I think at least total minimum of 5 hours.