Some CMC powders give a year's shelf life.  I have had some for several years with no problem, providing it is kept totally dry.
Mixed with water, I store it in the fridge (very clearly labelled . .even though it is non toxic!!!) and found it as good as new after two or three months.
Basically, if it seems alright, it is.

If your CMC/MC powder is in something that is moisture-permeable, like cardboard, I would put it in a heavy-duty plastic bag to extend it's life as long as possible.

Also keep in mind that there can be a very wide difference between real expiration dates (when the product is no longer useable), what the manufacturer thinks the exp. date should be so you'll feel forced to buy a new container of it, and what is required by law (if any).

Here in the U.S., the pharmaceutical companies and pharmacies are apparently required by law to indicate that all medications (prescription and over-the-counter) be labeled to show that they 'expire' in one year. 

Studies by independent laboratories have shown that many meds remain perfectly good for up to 30 years, although some 'families' of meds, particularly antibiotics, do have a much shorter lifespan.

I suppose one must get rather cynical and try to determine if the expiration date is valid due to physical characteristics of one or more of the the ingredients, or if it has been 'determined' by the profit motive.

Personally, I keep craft materials until I mix them and they fail at their job, or I can't get the acrylic paint out of the tube because it is so hard that I can't dent it with a large hammer.  One material appeared to be dated '1902', but I'm sure that couldn't have been correct.....

Give your materials a quick test before beginning a project.

Cynical Sue

I mixed some MC last year and used it to make a small cat shape.  I just had the basic lump-shape done, and left it to dry.  When I came home from work, the cat-lump was gone, the waxed paper it had sat on was under the table.  All four live cats gave me their most innocent looks and swore they hadn't touched it.

I found it a few days later in the crevice between the pantry and the stove.  The amount of lint and pet hair that was firmly attached to it indicated that they must have attacked it shortly after I walked out the door.  It just never looked very good after that, and it died a fiery death in the wood stove.

Anyway, I needed the kitchen counter for some other project, so I set the bowl of liquid MC on a shelf and promptly forgot about it.  It must have been a month later (maybe two) when I discovered it again, and it had mostly dried to a kind of dusty, rubbery disk.  I didn't drop in into water to see if I could reconstitute it, mainly because I didn't think of it.  But it wasn't moldy.  And if that wasn't a test, I don't know what would be.

Sue