Depends on how excited I am about a new project, how many figures need to be painted, how many buildings and terrain items need to be built, and how much disposable income there is. It can take anything from a few months (like my samurai skirmish stuff) to years (like my zombies). Of course, I also have plenty of lads who have never fired a shot in anger. And no project is ever complete because there is always one more building to make, one more unit to paint, etc.
As an example, I have been wanting to do 6mm moderns for probably a year and a half, maybe 2 years. Will finally be placing my first figure order tomorrow and have started making my first ever 6mm building. I anticipate taking about 6 months to get everything game ready (staggering out purchases and mixing up the painting with 28mm Sudan, 28mm zombies, 40k stuff – I get burnt out if I paint one period/theme without a break).
Those are brave men knocking at our door. Let's go kill them!