This all depends on the company, the project, and how close you are to pending deadlines.
Some companies will overwork their engineers which can lead to 60-80 hour weeks normally; the majority do not! Most companies adhere to the standard 40hr work week.
The occasional project is more involved than others as well. On projects where I've been learning new technologies, I find myself working an extra 5-10 hrs per week just to learn new things because I'm excited.
The other factor is how close you are to a deadline. We have all pulled a late night right before the big test. Nothing is different in the mobile app world.
To summarize, expect to work 40-50hrs per week on average. I will admit that some weeks I've worked as little as 30 hrs so it normally works out in the end.