Hi there. Well I'm in a similar situation. I'm not a profesional although I'm making a game for this years university proyect. I'm in production right now, the script is done and so is a storyboard. After making these two things I looked at the storyboard and estimated about 2-3 hours (is the first chapter of a game), but of course I will not know that until is played. I don't think rooms make the difference at all, you can have 40 rooms and go by them in half an hour. At least in my game, dialogues and puzlzes are what really extend the time (I have plenty of dialogue). And also there's the individual pace of each player. If you want my advice, try to think about making each part of the game an entretaining and powerful one, and do that until you can in your time. If in the end you have to make a few changes to be able to finish your game, then do it, but don't sacrifice art and gameplay quality thinking in extension. In my opinion is better to have little but great than lots but mediocre, so if you are a hobbiest and this is not your way of making money, then take all the time you need to find perfect each part of the game you have done. Still, in general, I think 16 rooms (16 rooms of doing something else than walking by) can fill about 3 hours... even if you don't have to go back so much. IT is always nice though to have a few rooms that are just for ambientation, and siometimes you can go back a lot to a room but make changes to it (it's darker, empty for some reason, whatever fits the story that can help not going through the same exact room that much).
Well I don't know if I helped in any way XD
good luck on your proyect.