Once you’re in the car park you can get out any time. This time of year it closes for entry at 6pm (8.30pm in summer).
Elsewhere, there’s Coral Reef over the road, which has been noted currently have signs up saying parking charges are suspended until further notice. Not sure if they have a closing time for the car park or it’s open all the time.
Otherwise there are various places around the forest. Nine Mile Ride has yellow lines along, and don’t try to park in the bay on the corner by the main trail entrances, they actively ticket people parking there. There’s further along up Foresters Way, little entrance off a roundabout with a kebab van often there. Known as Dog Shit Alley, for obvious reasons. Free but usually busy. Likewise up from the Dive Centre round by Crowthorne and ride in up Devil’s Highway. You kind of need to know where you’re going really. Also various spots along the Bagshot side.
Basically any entrance point to the forest, but just watch out for parking restrictions and never block a fire entrance.