I would like to know how to estimate the time it will take to paint interior walls, ceiling and trim.

There are many factors that would have to be considered. Do you mean for a professional painter to paint or is it DIY? How many people working on it? Same color on the ceiling and walls or different? Is the trim new or is it a repaint? Same color or different if a repaint?

I base the amount of time on the condition of the room and surfaces and the sq footage of the room. EX: A 10x10 room just needing repainting ceiling and trim all the same color.$2.00 a sq ft. = $200

Just me working as a part-time handyman for customer. I will tape old paper to upper walls to prevent spatter & maybe use duct tape on large stone fireplace. Don't have ceiling dimensions yet. Besides tape preparation time, can I multiply a certain factor X sq. ft. Bill

Depends on how fast and efficient you are. Roller or sprayer? I usually charge by the hour, they supply the paint.