Due to the efficiency of the insulation you may not feel any perceptible change in the warmth being given off for several hours.
Every AGA is individual and therefore they can take slightly different times to heat up.
AGA recommends the use of Class C2 Kerosene. However, suitability of oil should always be discussed with the fuel supplier. A number of oil suppliers do recommend additives for vaporising appliances. AGA only recommends Total Glomax which should only be used once the tank has been cleaned and flushed prior to its use.