Cooldown on hacking the same portal?

The hacking delay is personal and it is always 300 seconds long. The message shown though only changes when you have remaining 240, 120, 60, 30 and 10 seconds. So, if you keep trying to hack in the first minute, you'll keep getting the "300 seconds remaining" message.

All your successful hacks will trigger this 5 minutes delay. On top of that your first Hack triggers a 4 hours cooldown. After your 4th consecutive successful Hack on that portal it will be "burned down" and will prevent further hacks until the four hours have passed. This cooldown too is personal, to prevent its use as an attack (burning down all the portals in an area to prevent enemy hacks)

Adding a liitle more of information @Kappei's answer,

-Other Time that show is 5 secs. -Sometimes, if you hack, and just when the bounty appears, click on "Accept" and hack again, the portal will give you another hack. In my "team" we saw 6 hacks one after other. But the usual is only to have 1 per 5 minutes. Just try, what I said, because, sometimes you can get some surprise.

-With the last update in few days Niantic will leak the new Mods (5 in total), one of them allows you one hack more than usual (4-->5) and other downgrade the 5 minutes waiting to 4. (1 minute less)