This is the only cheat that i've been able to locate about this and it's an iffy code so it probably doesn't work but i figured i'd let you be the decision maker.
Unlimited time
INVALID CODE??This code was reported invalid by Users-You Decide
Pause the game in any mission and press Square(3) Circle, X R1, R2, L2, L1, Triangle, Select then you should hear a gunshot and then unpause and in the next part of the level (next part with timer) the timer should be stuck at the starting amount minus 1 millisecond have fun.
I hope that this bit of info was of any help to you or anyone else who may use it. Well, enjoy posting and have fun gaming. :thumbsup: