Durability gear

Hi!

I don't know if this is the right place to ask this but I would like to see that the durability of gears would also include for example a nr. of hours used instead of only distance. 

For example: I would like to add my tennis racket but it needs new stringing and a new grip after a certain amount of hours played. Now the only option I have is after a certain amount of km which is weird.