What is is strength of the key derivation? Specifically, how many milliseconds or microseconds would it take a desktop computer to check a guessed password to see if it is a match? I am aware this varies by computer, so I want a ballpark, and I'd give a 3GHz single core computer as an example, but take your pick.
Assuming there are no significant flaws in the implementation, and the symmetric encryption is of good quality, then I can use your answer combined with knowledge of my password strength to estimate the cost for an attacker to decrypt my Android.
Obviously there are caveats. If the phone is still on, the attacker could follow a procedure to bypass the encryption obstacle (using a freezer, and swapped ROM, or other technique). There may be other caveats worth mentioning.