There is no telling how it takes. Your chances of finding a Shiny Giratina in Diamond are 1/8192, so basically you should only expect to find one Shiny Giratina with every 8192 you encounter (though if you are lucky or unlucky, you may encounter them more frequently or less frequently).
Since this is entirely dependant on luck, it is impossible to tell how long exactly it would take. However, we can basically confirm that it will be later rather than sooner; I wouldn't say that odds of 1/8192 are in your favour.
Basically, just keep persisting. It is stupidly rare, so don't expect to find one early on.
