It'd be for 1 year minimum. And of course it is legally binding, these are the terms. Generally speaking, the only things that are not legally binding are contracts that involve illegal activity or contracts with a minor. Everything else can be enforced.

To address the arguments below...Elon Musk hasn't drawn up a contract because the offer has not been accepted. Of course he would draw up a basic, legally enforceable contract with these terms...if Wikipedia accepted, and said yes, I want $1B. Please draw up contract. This is how the world works. 🙄 Contracts, legal obligations, ability to enforce them.