Object Rotation By KeyDown

Hello, I have an object and I want to rotate it on one axis when a key is pressed.

I know how to do this but I want to implement a way to control the speed of the rotation by key press.

Concept:
Variable - RotAmount

So this will be a float that will start at 0.

When I press T, it will increase the RotAmount to 100.
And G will decrease it all the way to -100.

What I have is a fairground ride that you can control. So for example 1-100 would be rotating one way and -1 to -100 would be the other. And 0 would be not moving. This number hopefully would then change the speed of the rotation. If anyone knows how I could implement this then that would be great, thanks!

using UnityEngine;
using System.Collections;

public class RotationLimiter : MonoBehaviour 
{
	[Range((int)-100, (int)100)]
	public float rotation = 0f;					// rotation
	public Vector3 angles = Vector3.forward;	// angles
	public float keySpeed = 25f;				// speedup/slowdown speed

	void Update()
	{
		// change rotation variable
		if(Input.GetKey(KeyCode.T))
			rotation = Mathf.MoveTowards(rotation, 100f, Time.deltaTime * keySpeed);
		else if(Input.GetKey(KeyCode.G))
			rotation = Mathf.MoveTowards(rotation, -100f, Time.deltaTime * keySpeed);
		else if(Input.GetKey(KeyCode.Space))
			rotation = 0f;

		// rotate
		transform.Rotate(angles * rotation * Time.deltaTime);
	}
}