Increasing speed over time

Hi all, I have a game and I want to increase the speed of the ground overtime. I have the following code but I'm not sure how to specify how long to take before it increases it's speed?

Code (CSharp):

1. using UnityEngine;
2.  
3. public class Ground : MonoBehaviour
4. {
5.     public Material material;
6.     public float speedMultiplier = 0.5f;
7.     public float xVel = 0.1f;
8.     Vector2 offset;
9.     void Start()
10.     {
11.         material = GetComponent<Renderer>().material;
12.     }
13.  
14.     // Update is called once per frame
15.     void Update()
16.     {
17.         offset = new Vector2(xVel, 0);
18.         material.mainTextureOffset += offset * Time.deltaTime * speedMultiplier;
19.     }
20. }

 

I think this thread may be of use...

https://forum.unity.com/threads/question-how-to-gradually-increase-speed-of-movement.98067/

 

Thanks for the reply, but what i'm making move faster overtime is the background. the character is just running on the spot, and I set the background on repeat. I assume the thread above is used for making an object gradually increase speed?