How early can you arrive to a MLB baseball game?

more specifically at a san francisco giants game, i wanna get pictures with some of the players and i was told you gotta get there super early for their batting practice...so exactly how early should you get there and i was told that going to the dugout would be a good option. can you go to the dugout area even if your seats aren't in that area? & depending on the player, is it gonna be harder to get their autograph or picture if it's the more popular players like buster posey, tim lincecum, or pablo sandoval?