There is no hard and fast rule for this. Ferry bookings can be made any time, even up to the day of the travel. However, if you wish to book in advance for your own peace of mind, this is fine too.
All ferry companies differ about when you can actually make a commercial ferry booking. These can range from 2 months to 7 days in advance. It all depends what route you are looking for. If you need to make a freight ferry booking in advance we will keep the booking in our Freight2Ferry system and process it at the earliest possible opportunity.