Unexposed, coated screens should be "exposure ready" for as long as 6 months. This is assuming that you are storing them in an air tight container that keeps them free from light and humidity.
Screens that have been stored for too long will be difficult (if not impossible) to expose. They will have to be reclaimed and re-coated with emulsion. For a tutorial on screen storage, visit: The Perfect Solution for Storing Screen Printing Screens
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