The butter will go through several visible stages over the next 45 minutes to 1 hour. In the first 10 to 15 minutes, the water content in the butter will begin to evaporate and you will see gentle bubbling on the surface. The liquid will look opaque and pale yellow at this stage. Stir every 5 to 10 minutes to prevent the milk solids from sticking to the bottom of the pan and burning.
Between 15 and 30 minutes the butter will become clearer and you will begin to see the milk solids separating and settling toward the bottom of the pan. The surface foam will reduce. The colour of the liquid will begin to deepen toward a warm golden amber.
Between 30 and 45 minutes the bubbling will reduce significantly as most of the water has evaporated. The liquid will be largely clear and deep golden. The milk solids on the bottom of the pan will be turning a light golden brown colour. This is the most critical stage. Watch carefully and stir gently. The smell at this point should be deeply nutty, toasty and extraordinarily good. The moment the milk solids on the bottom of the pan turn a light amber brown and the liquid above is clear and golden, the ghee is done.
Remove from the heat immediately. The residual heat in the pan will continue to cook the milk solids for a moment after you remove it from the burner, so err on the side of taking it off slightly early rather than late.