A good rule of thumb is to plan about 1.5 to 2.5 hours in front of the camera for out-and-about photos. After that it gets old. Just because you have four hours between ceremony and reception doesn’t mean you have to use it all for photos.
Don’t forget travel time on top of the above mentioned estimate. You might be surprised how long it takes to get a wedding party on and off a bus, and traffic is always worse than you would otherwise expect.
Have a big wedding party (six or more per side)? Add more time. Big groups take longer to set up and get organized. Have a heavy drinking group (you’ll know if you do), add a little more time.
It’s a good idea to pick a couple good spots and milk them for all their worth rather than trying to hit every place you can think of. For example, consider two or three spots in Forest Park (MUNY, waterfall, Jewelbox) rather than trying to go to five, six, or more all over the place. You’ll enjoy a more relaxed pace.
Remember to give your photographer the time (typically every new spot will require some adjustment to settings and lighting) and attention it requires to get groups set up and make any adjustments to equipment. We all want to have a good time, and the photographer should keep it relaxed, but when it’s time for around-town photos it’s important to keep focused.