The faster my character goes the longer he jumps.

Are you using the AddForce method? If so, what parameters are you sending to it?

This is expected behavior from a Physics or $$anonymous$$athematical perspective of course. You have a few options I can see to work around this, each with consequences:

1. Calculate how much force to apply to always land X meters from the start of a jump - jumps will be lower the faster the player runs

2. Increase (or supplement) the Gravity force the faster he runs - player will go up and down faster which may look a little odd at really high speeds

3. Always set the speed to a constant value whilst in the air - player slows down whilst airborne

This is as much a design decision as anything... you may even decide to stick with the current situation after evaluating the options.

Could you give me an example please @Huacanacha

It would be great if you could include some of your code with the question, so we can see exactly what needs to be done.

Just get the velocity of your player by rigidbody.velocity.magnitude and calculate your desired speed at a point and make a if statement. If velocity is greater than your desired speed add a drag to your player or add a counter force. Hope this helps