If no monthly payment is due, then when do I payback the loan?

The loan is due when a triggering event, or maturity event occurs.

These include:

When the last surviving borrower dies and there is no spouse remaining in the home

When the home is sold

If the home is vacated for 12 consecutive months or more

Failing to pay property taxes or insurance

When the home is no longer deemed habitable as defined by HUD.