What should I put for "Approximate date you permanently left / intend to permanently leave the US."

If you are still in the US and do not have an exact date for when you expect to leave the country, here are the dates you can use: 

1. Your VISA's expiry date. 

2. Your DS-2019/I-20's end date.

3. Any date in the future. 

Entering any of the dates above does NOT necessarily mean that you would or must leave by the date mentioned. The date is used for the IRS to have an idea when you plan on leaving the US. However, if you change your plans and leave later or earlier than the mentioned date, that would NOT cause any issues.