Setting time of the jump

I have this simple script that makes player jump when he starts pressing space.

For now player can jump for a certain height, the problem is as height gets longer, jump time also gets longer and that's not what I want, I want to make that time constant, no matter what height of jump is and no matter how strong gravity force is.

There is a trick to decrease jump time by increasing gravity when player has jumped, but that makes more problems then solutions, jump height will be shorter and I don't know how much should I increase gravity force to make the time of the jump to be a value determined by jumpTime.

Here's the code (part of it):

 using UnityEngine;
 using System.Collections;
 
 public class Player : MonoBehaviour {
 
 public float speed      = 1.0f;
 public float jumpHeight = 1.5f;
 public float jumpTime   = 1f;
 public float gravity    = 10f;
 
     Vector3 InputAxis = Vector3.zero;
     Rigidbody rig;
 
     //First frame
     void Start() {
         
         //Init rig
         rig = GetComponent<Rigidbody>();
 
         //Disable Physics.gravity on this object
         rig.useGravity = false;
 
     }
 
     //void Update() {...} //sets the inputAxis value
 
     //Physics frame
     void FixedUpdate() {
 
         //Initialize velocity change vector
         Vector3 velocityChange = Vector3.zero;
 
         //Calculate jump velocity
         //InputAxis.y is only 1.0 when space is pressed down (GetKeyDown), otherwise it's 0.0
         velocityChange.y = Mathf.Sqrt(InputAxis.y * jumpHeight * 2 * gravity);
 
         //Add velocity change
         rig.AddForce(velocityChange, ForceMode.VelocityChange);
 
         //Add gravity force
         if(velocityChange.y > 0)  //If player has jumped
             rig.AddForce(0, -gravity * jumpTime, 0, ForceMode.Acceleration); //not working...
         else
             rig.AddForce(0, -gravity, 0, ForceMode.Acceleration); 
 
         //Reset input axis
         InputAxis.Set(0, 0, 0);
 
     }
 }