It's hard to say how long the outsoles will last, because it depends on the users weight, gait, mileage, walking surfaces, etc. I've heard of anywhere from less than a year (these were replaced for free under warranty), to outsoles that are still going strong from our first batch in 2013 (these are likely not worn very hard).
I can tell you our Classic Collection outsoles are made of polyurethane (PU), which is more expensive than rubber, but lighter and more durable.
Our new Aurelius Collection outsoles are made of thermoplastic polyurethane (TPU), which is 42% more abrasion resistant than our PU.
Lifetime guarantees have disappeared from footwear, and for good reason, because footwear endures a lot more wear and tear than any other apparel. Our 365-day guarantee is one of the strongest in footwear, and as far as I know, it's stronger than any of the other barefoot brands or dress shoe brands.