With an iPhone? It's only 1,000 possible PINs for that lock.
Which is why they offer the passcode option for something longer.
For Android, depending on ROM, the pin can get insanely long and you're pretty much boned.
You need a ROM to get a long passcode? LOL. You can have an "insanely long" passcode in iOS simply by disabling the "simple passcode" option. I just typed in over 100 alphanumeric characters and it was still letting me type more when I stopped.