poly = crap
go with factory original rubber, not cheap but will outlast the poly and won't squeak either. plus, better articulation (rubber moves poly is hard), smoother ride and black looks better than the gaudy colors.
yah, yah, "poly lasts forever, the best stuff out there" give it up, that is steer manure.
i would have to say it is more the enviroment the bushings are exposed to than the mileage experienced. if all you do is dry highway driving the bushings will last much much much longer than an off road trucks bushings due to the wet and dry and severe twisting force. if yours are gone, i believe it. we both are hard on our trucks. stick those pretty little nylon bushings in and watch them grind down.
i wouldn't say don't run the nylons if they are part of a kit. i would say to buy just the springs and u/bolts and buy the shocks and busings seperately. downside is the cost. a kit is complete and relatively cheap. buy parts seperately can come close to doubling the cost.