It depends on from which port you are going. If you are going from Padang Bai, it's short, only 1 hour and 30 minutes. If you choose a port in the south of Bali (Sanur, Benoa, Serangan), it usually takes 3 hours. Some ships also stop by in Padang Bai for more passengers. Shortest is the ride from Amed (Noth of Bali) it is just an hour.
If you are going to the port by your transportation, please have in mind traffic in Bali is very slow and sometimes there might be a traffic jam, because of the ceremony. So always have reasonable time reserve. If you want to use pickup by shuttle bus, they usually pick you up at least 2 hours before boat departures.