I cannot answer the question how long the silver coating will last from my own practical experience, as I am new to this. Time will tell.
The short answer must be “it all depends.” Having purchased a decent quantity of all the chemicals required, the cost of each re-coating is small.
I have found this informative article on the Sheffield Assay Office website:
https://www.assayoffice.co.uk/news/ever-wondered-why-does-silver-tarnish
They surely know what they are talking about. Clearly their advice is not specific to telescope mirrors but the priciples will be the same.
Presumably anti-tarnish paper could used to line mirror covers. The paper is available on Ebay in small quantities. The jeweller in a local town can order in bulk, by the ream. My dictionary tells me a ream is 516 sheets! I don’t yet know at what cost.
I have a recollection of reading in an old book that alum paper will delay tarnishing. The item included a recipe for making alum paper.