The longest I have kept RI is 3 months, simply because I used it up. Make a largish batch and remove what you need to use. Pour the rest into a glass jar and place a cut to fit parchment circle on the surface of the RI. Screw top on tight and store out of the way in the fridge. It will separate, so be sure to stir or whip before you use it again.
Keep in mind that the consistency of RI does change with time. You wont be able to use it for certain things.
As for how long it can be kept, in the books it says is best up to a month. What happens is that the icing
will separate and you will have some liquids at the bottom of the bowl.
I dont like it after this happens I am too picky, but some people remix and use.
Edna 
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