This works for me. It requires the KeyChord library:
;;Exit insert mode by pressing j and then j quickly
(setq key-chord-two-keys-delay 0.5)
(key-chord-define evil-insert-state-map "jj" 'evil-normal-state)
(key-chord-mode 1)
It is inspired by @phils answer above and based on Simon’s Coding Blog: Emacs and Unity Every Day.