Is anyone aware of testing requirements for PPE clothing such as Flash Suits, etc?

There are several ASTM standards
ATSM 1449, 1506, 1891, 1958, 1959, and 2178, to name a few.

What about testing after months of use? How do you verify the clothing remains flame retardant?

You dont, there is no way to test them. This is where the manufactures laundering instructions become so important, follow them to a tee and replace them after the # of washes the manufacture says the rating is based on. The actual ATPV of the clothing when it is new is much higher than what is on the label. The rating takes laundering into account, but you have to do it right hich is why I dont trust a uniform company to do mine.

Laundering Instructions

Just a food for thought, it really depends on the type of fabric clothe you or your employees are wearing. Because there are alot of fabrics that tend to withstand the arc flash chemical than others. For example Nomex compared to FRT cotton. But if you want to increase your knowledge feel free to use texsoinstruments.com as an additional resource.

Not much to learn there, and there are some major misconceptions. I looked around that site, seems like a small distrubutor site with inflated prices. Not sure what anyone here is supposed to lean from this site.

Texso has alot of great information pertaining to the Arc Flash industry including products & training videos.