It’s difficult to know for sure how long earthbags will hold up in sunlight. Some brands fall apart pretty fast, as soon as a few weeks. Equatorial and high elevation areas may experience slightly more rapid deterioration due to increased UV levels. From my experience and what I have heard and read most bags hold up okay for about 2-3 months. The safest way is to buy tarps or black poly and keep them covered as much as possible.
However, tarps are prone to blowing around in the wind and can be a bit of a nuisance. If you’re doing a large job, then you can either buy UV resistant bags, which cost more, or in the case of vertical walls do one wall section at a time (including adjacent corners) and apply at least one coat of plaster as soon as possible.
Note from Kelly: One good indicator of how long the polypropylene material will last if exposed to the sun are the tarps themselves, which are often the same material. My experience is that they just get progressively weaker as time goes by, and may show signs of deterioration after about two months’ exposure and usually by about 8 to 10 months it is pretty easy to punch holes in them with a finger. And these tarps are often supposedly UV resistant. Obviously the best thing is to not take chances and keep the bags covered at all times, except when working on them.