Lets do some math fellas!
The longest distance possible between earth and mars is 401,000,000 km. The speed of light is 299,792.458 km per second. We divide the distance by the speed of light and we get as my calculator says 1337.59 seconds (not joking its leet) so it would take about at longest around 22.283333333 minutes to send a message from planet to planet. But remember at some times the sun will be in the way so for part of the year you can't even send messages!