The departure information sent with your tickets etc. will tell you this.
If it was me .. I would be on the ship around 1 pm see my cabin then have lunch and a look around the ship ... then enjoy sailing away ... no rushing which is what a cruise is all about.
Normally its an hour before sailig, but check your embarkation information, remember ship's don't tend to wait for you, if your lat.
The Cruis line will send you information on what time you can board ship. It is all staggered times to save congestion. I recently boarded the Ventura at Southampton and from leaving our luggage to check in we were in our cabin 20 mins later. Excellent organisation.
Usually 2 hour.
All Cruise lines will send you embarkation times along with your tickets. If you are flying your flights will coincide with your embarkation time.
We arrive at the terminal at approx 12 noon and we have always been on board by about 1 pm at the latest.
We sail on Summit to Hawaii from San Diego last year and only got there 1 hour before. This was great, no lines and were taken direct to our cabin. hope this helps.
When you get your tickets they will have a time for you to arrive, have been in my cabin 20 mins after leaving car once, but the wait is not long if they put you in the waiting area, way shorter than waiting for a 'plane,
enjoy your cruise.
Normally 1 hour --but this is cutting it too fine.