The only carrier I know of that is degradable is the CO-OP, on which it states: "From date of manufacture, the plastic will start to degrade in approx. 18 months time. The whole process will take about 3 years"
We use bio-degradeable plastic bags here, and I have known someone to put loose rice in one in a cupboard and a month or so later pick it up and the bottom fell out, so maybe they weaken long before they actually start to disintegrate.
It should be noted, though, that the latest type of carrier bags start to degrade even without being placed in soil. Our local council refuse to accept carrier bags in the recycling bin because many of them have degraded too far by the time they get to the sorting process.