For most compostable cups, the outer shell will break down within a year. However, the inner plastic lining will not. Oftentimes, as the plastic lining breakdown, it contaminates the surrounding compost. And for a backyard compost, plastic compostable cups usually don’t break-down because the temperature never gets hot enough. And unfortunately, compostable products used at a commercial level often don’t break-down because only certain disposal facilities offer composting. So, the cup either goes to the landfill (where the conditions for composting are not correct) or the waste has to be transferred to another disposal facility which can be towns, cities, even oceans away.
Compostable cups are a valuable method to reduce the impact of single-use items but are not sustainable due to a lack of sufficient disposal facilities. That is why the best method is to bring your own, reusable cup.