The "Mine" Stage
My daughter is learning ownership. Everything is Autumn's. A great development milestone. Sharing at this age isn't a toddler's strong side, but it's a learning one. Toddlers don't have an understanding of time at this stage, so asking to hand over something that is not theirs in 5 min is not going to work. To teach your toddler to share, ask her/him if you can have a turn. Going back and forth with this will teach your toddler to share.
I agree with you about the time. They are too young to comprehend that. We applaud Jonah when he shares which encourages him to want to share more.