When running this assay in Megazyme, we use a boiling water bath which allows the temperature of the solution to reach between 95-100 degrees quicker.
We suggest placing the beakers in the 95 degree water bath and allowing the sample/buffer to reach a higher temperature of approx 60 degrees before addition of the alpha amylase.
This will should lower the time it takes to reach the 95 degrees. Once 95 degrees is reached you can start the 15 minute timer. The longer duration will not cause an issue. The importance of this step is for starch gelatinisation and hydrolysis by the alpha amylase.