It depends on the type of service. Which cruise line are you going on? Most mainstream lines have gone to automatically adding a fixed amount per day to your onboard account which covers your cabin and dining staffs. All bar purchases have 15% automatically added. Room service is typically tipped in person $2-$3 when they make a delivery.
Generally the guidelines are $3.50 per person/per day for the waiter and cabin attendant (each get $3.50 per day), and $2 per person per day for the assistant waiter. Thus for a couple going on a 7-night cruise the total tip tab for these crew members would be $126. Most people like to give more since the service is so wonderful. If you have a suite, some lines up the suggestion for your cabin attendant to $5.75 per person per day.