Claiming refund from property developer for delay in handover

I had invested in an off-plan property with a developer paying 30%.

A year back, they offered me some discount to pay the balance (70%) in advance of handover mentioning it will be in May 2018, which I foolishly enough did.

The official handover date was March 2018. So it is delayed by more than a year grace period and I have not been notified in any way about any delays. They are no longer responding to emails.

The project shows completion of 82.18% at the land department website, but going by construction on-site, looks like it will take another 6-9 months, if not more.

Questions:

- Can I claim a full refund for money paid with interest?

- How much time does the legal process take because I have been told by some friends that the developer can drag the case for years?

- Any other option that I have as an investor to deal with a powerful builder?

Appreciate any advice that you can offer. Happy to share the documents for review.