There isn't a hard and fast rule, obviously what is important is to make every minute count. There is no point making a 15 hour course of content that students take and feel could have been taught in 3 hours, this will lead to low ratings and long term won't work as a strategy.
However, if you have 15 hours of great content that teaches what the students want to learn then this will be more successful.
One thing is to look at what competitors are doing, do they have short courses of a project or two per course, do they have courses containing 10-20 projects? What is the value to the student of more projects vs fewer projects or could they learn what they need to learn from fewer projects?
Would a student want to learn everything in the course, or would they be taking the course for a specific bit of learning and would a different student be after a different bit of the information?
Generally, although again, not a hard and fast rule, longer courses sell better as long as they contain useful content the student wants to know throughout. For example, I've just finished taking a piano learning course on Udemy, it taught lots of things which could have been in separate courses, so it could have taught the basics and then the section on Jazz piano could have been a separate course, the same with classical piano, blues piano, etc. But it was all in one long course, so I chose the long course because I knew ultimately I would want to work through all of these things, so I would rather pay one price for it all than pay for the basic course, then for the other courses separately when the cost on Udemy is usually the same regardless.
So if your competitors have everything in a long course then when people see all the courses at $9.99 each and they see a long course covering everything they want to learn, they will likely take that course because it has everything in it, rather than pay exactly the same price for what is essentially the first module of the long course but by a different instructor.
It could be that the short course is actually better presented etc, but people do consider how much content they will get for that same payment. If both courses claim to teach exactly the same content but one can do it quickly and the other takes hours, rather than having additional depth to it, then people would be likely to choose the shorter course and learn what they need to know in that shorter period of time.
All the best
Dan
