From an employer's point of view, you are leaving them in limbo without adequate updates and no awareness of if you will ever go back. Businesses are vulnerable and need to plan ahead for staffing issues, and to have someone off sick for over a year is a major strain.
It is not necessarily about what it costs them but about the functionality of the business and department. If you don't see yourself returning in the next 12 weeks, I would say it is time to resign.
I was once in a situation where I was off ill a fair bit and then in addition, my dad died so I was off to address the probate and sell the house (work was five hours away from where I was needed, back where my parents lived).
I took the pragmatic view that I could not keep work hanging on without me knowing when I'd go back, so I resigned right away. They then did the decent thing for me too, and provided redundancy pay even though I was not redundant. Think of how you can be fair.
I also saw my ex-partner keep his employer hanging on for 18 months due to his mental health issues, and it turned out he had no intention of returning. He had not provided them any updates at all, just saying 'I am still receiving treatment.'
It caused a major problem in the construction business as he was the site manager.