I looked into washable furnace filters last year.
First, they are not permanent. They're good for a few years - but they can't tell you how many.
They flow less as they get older - this can be a bad for your furnace. They flow less than a disposable filter even when new.
They don't filter as well as disposable filters - not good (enough) if you have pets or are bothered by pollen.
They need to fully dry when you wash them. You should keep a disposable filter for when washing.
Some should be washed every month.
I buy the cheapest brand name (that I recognize) disposable filters that the 'big box' store has. If I pay more for a 'better' filter, I'm tempted to not replace it as often as I should.
I've had the same washable filter for 10 years and still going on strong. It is made of plastic so I'm not sure why it would degrade and only last a few years. I wash mine every month, the same time frame given to replace disposable ones.
My washable filter flows more than a disposable one since it is more porous and this makes it even easier on the furnace, not the other way around.
Because they are porous, the don't remove as small of a particulate so it would be a concern if you have allergies.
I can wash and fully dry mine in about ten minutes. Again, it is made of non-organic materials that don't absorb water. I can easily due it between heating cycles without putting in a disposable filter.