Function Graphics:TGraphics( width,height,depth=0,hertz=60,flags=0 )
As you can see the default is 60, which is probably the most widely supported. I meant if -1 is passed as the Hz (I wasn't talking about Flip, should have made that clear) as MGE said, it will use whatever the OS/Driver provides as default.
Note: Some video card drivers do not return a Hz param so you cannot properly search for a chosen mode with a fixed Hz with GraphicsModeExists().