Unfortunately this is not possible at the moment. However, when in the live chess you can put a game seek out for "999 999" time control which would be quite a long game with plenty of time. And then in the online chess (http://www.chess.com/echess/create_game.html) the slowest time control you can create a game seek for is 14 days per move.
How do you play chess without the timer on Chess.com?
If you need help, please contact our Help and Support team.

How do you play opponents without a timer?