My theoretical understanding is that the coating lifetime is influenced by environment (chemical action) and handling (mechanical action). So if your mirror is in a well-sealed enclosure and you never touch it for cleaning, I'm guessing that it won't need re-coating for decades. If you own a dob and you observe often (i.e. lot's of environmental exposure), then you will eventually want to re-coat the mirrors.
My experience with a couple small reflectors that were poorly stored for many many years is that the coatings lose their reflectivity generally and develop uneven defects. I had a 4.5 inch mirror re-coated by Normand Fullum and it made a noticeable difference at the eyepiece. That coating was in pretty bad shape, initially.
It would be fun to experiment with a few weathered mirrors and try and establish a 'before' and 'after' reflectivity. Something quantifiable would be best.
For the moment, I've been given the blessing to use the equipment until further notice. It was ear-marked for Carleton University, but it's been sitting in a warehouse for years. I'll try and restore it over the next few months as a side project. First step is "coat some kind of glass with aluminum so it looks shiny".
It's good to hear that there are at least a few locals who need things aluminized.
-Adam
