The park closing time currently varies depending on the season and daylight hours. It is 5 PM in winter and up to 8 PM in summer. You can check it (and get a good trail map) at co.sanmateo.ca.us
It opens always at 8 am. If you need an earlier ascent you can use the Old Ranch Road Trail (and avoid the parking fee) which will give you a 2 mile one way all trail approach.
San Bruno Mtn is not San Mateo County's high point, but it IS the most prominent peak in the county. It is ranked #53 of the 58 counties in prominence, as listed at Peakbagger on its california county prominence peak list
"After the first glass, you see things as you wish they were. After the second, you see things as they are not. Finally, you see things as they really are, which is the most horrible thing in the world."
--Oscar Wilde on Absinthe