The factory LS1 pulleys all suck, cheap crappy plastic ones. Are you replacing them with genuine or after market pulleys? The after market steel ones will be significantly better and should last a while.
Plastic or steel is irrelevant as both pulleys use the same type of sealed bearing..
Also. of the 4 idler and tensioner pulleys on an LS1, only 1 of them is plastic which is the main belt tensioner. The other 3 are steel. You can buy an aftermarket main belt tensioner in steel if you want, but why when it has the same type of sealed bearing as the plastic pulley? The only disadvantage with the plastic pulleys is that you can't press out their bearings when they go bad as they are retained in an integral collar that will break off, so instead you must replace the entire pulley.
Of the aftermarket steel pulley for the main belt tensioner that I have seen, it's a Chinese-made and I have very little faith in Chinese product, and sourcing a quality non-Chinese one still ends up with the same type of sealed bearing.
When I swapped my A/C pulley bearings, I deliberately replaced them with Japanese bearings. I believe the Nuline brand are OE, and if so, they've lasted 300K. The other 2 main belt idler and plastic tensioner pulley bearings are both still fine.
