Any juice inside of a closed tank should be absolutely fine to reuse. However, the tanks themselves will no doubt need cleaning if they've sat for a while. Coils in rda / rdta can be reused also but will need to be cleaned, dry burned and rewicked. Liquids evaporate. And more often than not, coils and cotton left for a while will dry out with a more concentrated gunge of sugars and flavourings left behind. Personally, I would drain any juice from closed off tanks into a bottle. Remove any cotton from the coils. Dry fire the coils. Rinse. Put thru ultrasonic cleaner with coils still in place. Put on paper towel and leave to dry. Put back together and check coils fire evenly. Rewick and reuse juice saved from earlier.
I've had juice soaked into the cotton go manky as it evaporates over time and leaves a sugery mess behind but I've never had to throw away juice from a closed tank just because it's sat around for a while.
I am talking about rebuildables tho. For tanks with stock coils I would replace the coil.