The short answer to how long do water barrels last is that they last a very long time.
55-gallon plastic water barrels, the water drums most people use for emergency water storage, are made of HDPE or High-Density Polyethylene. HDPE is a very strong, very stable plastic that is very UV resistant. We have personally tested HDPE 55-gallon water barrels in direct sun, stored outside year around, for over 10 years. The results of our 10-year test were actually amazing. We noticed a slight lightening of the blue drums, but the plastic was still in very good shape, not brittle in any way, and the water inside did not have any plastic taste to it at all.
We have had customers report similar results to ours for even longer periods, some as long as 20 years. Naturally, you want to store your water barrels in an ideal environment, but these barrels really hold up. In one research project conducted on corrugated HDPE pipe, it was found that HDPE materials lasted over 100 years.
It is our estimation, based on currently available information, that your 55-gallon water barrel made from HDPE will last you a lifetime.
This is a very important exception to how long water barrels last. During our 10-year test of HDPE 55-gallon barrels, we did find one problem that you should pay attention to. Bung caps do not last as long as HPDE drums. We found that after several years in the sun, bung caps became faded and brittle, which can make them very difficult to remove. We recommend changing your bung caps every 4 or 5 years if your drums are stored in the sun.