Something nylon like Kitty Nylon or Angel Hair will last forever. Kevlar string won’t wear down either. Graou String is the most durable polyester string.
Dragon lasts a nutty amount of time, as does normal Toxic. However, they play very differently from standard poly. You’ll have to fire up a 10-pack sometime I guess, in order to know how you like them!
Can’t compare to Nylon or Kevlar, as I’ve never tried either.
Type X is not super-durable. Most strings on the “softer” side have that as a trade-off. Just the nature of the beast.
Good go with quantity over durability and order up a bunch of bulk poly (or one step up with bulk Kitty) and just replace it as needed.