You can cook or bake with Lactase Drops once the enzyme has been left to sit on the dairy product at cool temperatures for about 24 hours. It is not recommended to heat the dairy product before the Lactase Drops have had a chance for full hydrolysis.
At temperatures between 95 °F – 122 °F, the enzyme activity is actually sped up, however, temperatures much higher than this the enzyme is not stable. According to studies on this product, the Lactase Drops are only stable to a maximum temperature of 131 °F. If you wanted to use the lactase on the milk before it was brought above this temperature, you could do so. If you were able to slowly heat up the milk, allowing the Lactase Drops to act on the milk at this temperature range, you could speed up the process somewhat. However, there is no real way to measure whether there is lactose still left in the product because there have not been studies to determine the length of time full hydrolysis takes at these temperatures. The safest way to achieve the level of hydrolysis you are looking for would be to allow the drops to act on the cold milk (43 F) for a longer period of time (24 hours). Increasing the concentration of Lactase Drops to milk can also help expedite this process.