I recently switched to a 15-character alpha-numeric/special characters passcode after reading an article by a security researcher.

A snippet from that article:

iOS estimated passcode cracking times (assumes random decimal passcode + an exploit that breaks SEP throttling):

4 digits: ~13min worst (~6.5avg) 6 digits: ~22.2hrs worst (~11.1avg) 8 digits: ~92.5days worst (~46avg) 10 digits: ~9259days worst (~4629avg)