It's been stored in a tupperwear container in my fridge for the last 3 weeks is it okay to use? How long can it last?

it depends on what you made it with. if you used butter and cream for your buttercream i would probably not use it. If you used shortening and water you are probably good to go. However, icing tends to start tasting like refrigerator... if you decide to keep it... mix it up, smell it, and taste it.

Agree with the above and will add that unless its crystalized and started to get hard from being in the 'frig, it should be ok.