How many hours does it typically take to play through the single-player campaign of Mass Effect 3 from start to finish on the default/normal difficulty level? How many hours does completing all the side quests tack on to that?
This is a question that no single person's experience can answer for you, as we all have different play styles, play on different difficulty levels, and play on different platforms. These can all have a large impact on overall play time.
A good answer will collect data from multiple sources and take an average. That's exactly what this page on HowLongToBeat.Com aims to do, and it also breaks it down by "just main game", "main game plus side quests", etc.
The average is looking to be about 23 hours, with another 8 or so for the side quests.
It takes around 30 hours to finish according to VideoGameLength