1 second of driving at your current speed should be minimum.
I'd make that 3 seconds. Longer at high speeds, if the roads are slippery or if you drive a heavy vehicle. You can shave that figure down a notch if you're in a long column of slow moving vehicles ("bumper-to-bumper"), but be aware of the risks involved.
1 second would make a serious violation of our local traffic laws and the ticket if caught is quite stiff. Less than 1 seconds and you're borderline to serious legal trouble (you go to court, not just a ticket) and loss of driver's license. Your local laws may differ.