well... both matter.

well dyed jeans(many dips in the indigo dye) will take longer than poorly dyed jeans. they also take on deeper hues once they start to fade. this is the reason japanese brands like sugarcane and samuari take a long time to break, and nudie gets great results in 6 months. this is one factor.

the second is the yarns used in the denim itself. if you have ringxring denim, the core of the denim is going to be white, with all the indigo on the outer part. with OE yarns, they are more porus, so more indigo leeces into the center of the yarn, making for a less vivid fade that takes longer to show contrast. most selvage denim is ringxring.

the person wearing also plays a huge role. if you just walk around someties and go out in your jeans, it will take a long time to show wear. THIS is the main reason that people suggest waiting 6 months to wash. if you are very active, you will fade your jeans much quicker. see my 2 month old sugarcanes in the evolution thread for an example of this.(i skatebaord 1-2 hours a day in them, and have washed them 5 times)