Cooldown is the time between individual hacks of a Portal, 5 minutes typically. (Not to be confused with Burnout, which is 4 hours). Cooldown time can be reduced by deploying Heat Sink mods to the portal, see Heat Sink#Cooldown Reduction for details. Deploying a Heat Sink also immediately resets the cooldown timer for the agent who deployed it.
Like burnout, cooldown is per agent; hacking never affects other agents' gameplay.
When hacking during the cooldown period, the following message is displayed: "Portal burning hot! Unsafe to acquire items. Estimated time to cooldown: X seconds."
The "estimated" time is given within one of the following intervals, rounded up: 300, 240, 120, 60, 30, 20, 10, 5. For example, getting "240 seconds" means you have to wait somewhere between 120-240 seconds until your next hack.