How fast a pool can be heated during the day generally depends on how many hours of direct sunlight your pool is getting each day. Therefore a solar pool cover needs to cover the pool for the available duration of sunlight which can increase the temperature of the swimming pool water by up to 15 Degrees Fahrenheit for approximately 12 hours.
If you are using a solar cover as the only means to heat your pool, be warned, it is not likely that your pool is going to gain as much heat as you are expecting. Solar covers help keep the heat that is provided by an external pool heater, they are not meant to be used as the main source of heating.
Evaporation is the main cause of heat loss in a pool, therefore having a solar cover is going to act as a barrier between the water and the air. A solar cover will help prevent evaporation throughout the night when the air temperature gets colder. A solar cover will not heat a pool overnight. A Solar cover will help pool water warm up faster with the use of an external heater.