I think it depends very much on how much engine you need. A simple renderer (in 2d or 3d) is a fairly low time investment assuming you are a strong C / C++ prog

I think it depends very much on how much engine you need. A simple renderer (in 2d or 3d) is a fairly low time investment assuming you are a strong C / C++ programmer. However every thing you add on top of 'draws stuff to the screen' adds at least the same amount of time again. Physics, animation, editor, UI, audio, streaming, packaging, asset import, networking. Plus each one of those new systems may require new features from the existing ones in order to work (eg animations will need asset import support, shader support, and renderer support).