Depending on the origional age of the unit itself and how its been treated,
I know that sounds like crap but I'll try to explan.
I work as a roving dealer for "Spectra Watermakers" wher I service
and sell the units..
Membrane life is affected by a number of factors and is somewhat unpredictable.
A big commercial
plant running 24/7 will get 10 to 12 years out of a set of membrains, but they do all kinds of fancy stuff with chemical injections and never shut it down.
Most cruisers are lucky to get 5 or 6 years out of one but you hear of 8 or 10 year old units still working well.
The worst thing for a watermaker
is to set doing nothing.. Biological growth will occur in the membrane and the unit will develope a rotten egg smell caused by hydrogen sulfide.
is produced by anaerobic bacteria that live in an unused membrane, feeding on whatever animal or vegetable mater that is trapped in it.
DONT EVER USE CHLORINE OR BLEACH to clean your watermaker
as it will destroy the membrane in a mater of minutes.
I recomend using Propylene glycol and potable water..
NOW FOR YOUR QUESTION, I recomend it can be used Safely for one year.
Given GOOD care, a membrane will eventully just start to slowly fade away.
The feed pressure may rise and/or the PPM will go up... hardly ever will they just fail overnight..
I have also brought back to life, a membrane that was pickled for 5 years but how long it lasted after that I dont know..
What I would suggest is to do a flush and clean the unit out, take a reading for the PPM and see where it stands.. Remember they love to run, so let it run for a few hours.. with luck, she'll come back to life.
Now on another note, My unit is hooked to an MPC 3000 that does a fresh water flush every 5 days or so.. and all I need to do to fire it up is push a button. if a watermaker is in your cruising future, you may want to think "upgrade"
Anything else, I'll be happy to help