Nice Airport toward Cannes by car will take you, basically:
35 minutes but according your travel period, you may double the time for this journey.
The Cannes Film festival and sometimes during the high season, it may be difficult while getting out of the freeway.
Usually, the driver takes the main road but you can ask him to shorten the ride with a shortcut.
Nice Airport / Cannes Downtown: 26Km (16mi)