I've honestly never grown mint from seed. They are so much easier to start from cuttings or pieces of root. Just ask someone who has it in their garden. They will insist you take some :) Sorry that doesn't really answer your question, but they sound pretty stunted for their age.

Forever. Or never, depending on the conditions it's in.

Mint is never fully grown. It just keeps spreading and spreading. Which it can't do in a pot, especially a teensy pot. It will choke itself to death instead, unless you re-pot regularly. Give it room to spread, and it will!

Check these out..

Here is a link that might be useful: Planting mint seed