As the day progresses, sunlight that penetrates through the clouds will warm the surface and the air above. The warming is greatest over land areas as land heats up much faster than water. As the air warms it is mixed upwards and will begin to mix into the clouds. This warming of the cloudy air decreases the relative humidity below the 100% level and the cloud begins to evaporate.
The time it takes for the marine layer to dissipate depends on many factors. Perhaps most important is the thickness of the marine layer clouds. A thicker marine layer will dissipate slower as it will take more time for enough warm air to mix in and evaporate all the cloud. It also appears that the moisture content of the air near the surface plays an important role. Relatively dry air (low relative humidities) near the surface will act to increse the rate at which marine layer clouds evaporate and dissipate, while relatively moist air (high relative humidities) will slow the dissipation process. The strength and direction of the wind can also play a role (see below).
The plot below shows relationship between the inversion base height (x-axis) and the thickness of the marine layer clouds for individual days during June-August 2005-2010. There is a strong correlation of R=0.76 between the height of the inversion base and cloud thickness indicating that thicker marine layer clouds tend to form as the height of the inversion base increases. Daily weather balloons (radiosondes) launched at 4am local time from Miramar measure vertical profiles of temperature and humidity and were used to calculate the top and base of the inversion layer. Cloud thickness was also estimated from the humidity profile by assuming a critical relative humidity of 95% (Cloud assumed present if RH>95%).