I have a problem regarding rotation of a model around its axis using the keyboard.

if (newState.IsKeyDown(Keys.Left))

This is a sample of the code used to increase the rotation. rotateY is a float.
The rotation is calculated using the following code:

worldMatrix = Matrix.Identity * Matrix.CreateRotationY(rotationY) * Matrix.CreateTranslation(modelPosition);

Whatever the value I add to be rotated it is still the same speed.

Are you saying that no matter what value you use for rotationY, it always rotates at the same speed?  If so, then I would assume that rotationY is not getting set correctly.  You should use the debugger or some WriteLines to verify that the value is changing as you expect.

As for what value to use, you can define a RotationsPerSecond constant that contains the number of radians needed to do a full rotation in one second:

float RotationsPerSecond = MathHelper.TwoPi / 60f;

Then just multiply that by the number of rotations you want per second to get the value to add to rotationY when the key is pressed.  The value you use in your example (.03) should produce a full rotation every 3 seconds.

Tom

L0STlogic:

The angle needs to be in radians, use MathHelper.ToRadians() to do it.

For example:

Matrix.CreateRotationY(MathHelper.ToRadians(myAngle));

Just a quick clarification on this -- you only need to call ToRadians if the original angle is in degrees.  If the angle he's using in his example (0.03f) is what he is actually using in his code, I would assume that is in radians already and doesn't need to be converted.  If that is the angles in degrees, he likely wouldn't be complaining that the model was rotating too slowly fast!  ;-)

Tom

EDIT: Fixed bonehead mistake.