I think you can spend several times that without "seeing it all", but I know I'll enjoy the amount I can see! St. Petersburg has been on my "to do" list for a long time!
I was there about 10 or so years ago, it was still pretty Soviet. The building I was in didn't have its heat turned on by authorities, so I froze. I bought a heavy army winter trench coat on the street for 15 bucks and slept in that.
Neat place, I'd like to go back, but it was a demanding, cold trip for me, from Phoenix to New York to Prague, spend a few days, then Helsinki, spend the night, a few days in St. Pete and another night in Helsinki and Prague again. All flights stand-by.