Cellophane itself is biodegradable.
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http://en.wikipedia.org/wiki/C...
But are we sure that the film over the envelope window is literally "cellophane"?
Cellophane is just cellulose, dissolved and then re-precipitated and washed, with glycerin added to keep it soft and flexible. By itself, cellophane film repels water but lets water vapor pass through. Since it's nothing but cellulose, it's chemically like paper without the lignin and binders ... it's all one sheet instead of thin fibers, but still just cellulose.
If extruded as a thread instead of as a film, it's called rayon.
It does repel bacteria, so I would expect it to break down somewhat slower than paper.
(Some cellophane is made impenetrable to water vapor by coating it with a little nitrocellulose lacquer. I would not think envelope windows are coated that way - it would be quite futile. Paper lets water vapor AND liquid water go RIGHT through! Despite the name, nitrocellulose isn't toxic. It's used in nail polish, playing cards and ping-pong balls. It used to be used as a wound dressing (collodion) and as lacquer on guitars.)
An easy test for patient people would be to cut those windows out of a few weeks' worth of bills. Then crumble them together with a handful of compost that's fresh and "cooking". Tuck the bundle into one part of a compost heap, or bury it in fertile soil, and mark the spot with a mini-blind slat or bamboo skewer. Check it once a month until the cellophane and the paper its glued to "dissolve".
If it breaks down slower than you like, shred it into slivers before adding to the heap. Until it breaks down, it's an inert fiber soil amendment. After it breaks down, it's fungus-food.