It takes an awfully long time just to get a basic framework that's reliable, consistent, and usable for simple demos.  I have been doing work on my engine for around a year now, programming during my free time.  It has just recently started to become decent enough to get some demo apps out of it.  I have been focusing mostly on the 3d renderer so far.  You need to make sure that your core architecture is solid and that it allows for simple and flexible expansion.  Make sure to keep track of documentation, especially if you are working alone.  This is one thing I regret a lot.  Most of the time I add new features, I need to look over old code and try to remember how it works.  I have not documented my code much and it is starting to bite me in the ass.

It takes an awfully long time just to get a basic framework that's reliable, consistent, and usable for simple demos.  I have been doing work on my engine for around a year now, programming during my free time.  It has just recently started to become decent enough to get some demo apps out of it.  I have been focusing mostly on the 3d renderer so far.  You need to make sure that your core architecture is solid and that is allows for simple and flexible expansion.  Make sure to keep track of documentation, especially if you are working alone.  This is one thing I regret a lot.  Most of the time I add new features, I need to look over old code and try to remember how it works.  I have not documented my code much and it is starting to bite me in the ass.