When can I withdraw the money from my SAYE account?

Anytime. But dont withdraw, your interest will be affected. Just hold for 2 years. Is the best "endownment plan", basically free money, risk free.

You can only do it once, so u want to max the amount contributed.

Iirc you can withdraw anytime you want. I opened mine on 3rd Jan 2018 and closed it in Apr 2019. The only downside is that the overall interest rate will be much lower since you didn't hold it for the full duration. But if you're using the money to invest then by all means go ahead, investments will net you higher return than SAYE