The industry says 3-5 years for ST (special trailer) tires. I assume your tires are ST type. However Westlake has P and LT tires also and had some recalled some time back.
Just because they "look" good doesn't mean much as you and many folks with large 5th wheel trailers and 80psi ST E Westlake tires found out.
Trailer tires don't carry a mileage recommendation/warranty and may last 6k-8k or 16k-18k miles on heavy trailers. If you don't know how to check for what to look for have a tire dealer with trailer experience check them out.