How long before debt written off

I have an unsecured debt with the back. (It was a write down from residual debt leftover after selling investment properties)

But now I can't pay the write down either. And the bank ain't for budging.

My understanding is they can no longer legally come after me after 6 years? Is this correct?

If so, from when does the clock start?

I sold the properties about 2 years ago. But agreed on the writedown amount a year ago - but reneged due to losing job. Does that mean in 5 years I'm in the...

How long before debt written off