There's nothing relaxing about watching the check-in clock while you're stuck in port traffic. Give yourself a little more room than you think you'll need.
The time that matters is check-in closing
A ferry departure time and a check-in deadline are not the same thing. Your operator and booking confirmation should tell you the terminal and when you must be checked in. Read that part before planning the drive.
Why I'd build in a buffer
Motorway queues, border checks and unfamiliar port roads are all easier to deal with when you aren't rushing. Allow more slack during school holidays or when travelling with children, animals or a large vehicle. There's no universal magic number of minutes.
If things are going wrong
If you're delayed, use the contact details on the booking to ask the operator what options remain. Don't assume that reaching the port just before departure means you'll be allowed onto the ship.
Routes to compare
A few things you might be wondering
Do cars have a different check-in time?
They can. Follow the vehicle instruction on the booking confirmation.
Should I arrive hours early?
Arrive for your stated deadline plus a sensible buffer.