It depends on a lot of factors. Your writing speed, discipline, how much coding you have to do, whether you're drawing art yourself or commissioning it, funding, etc. DD has probably around 4000 hours of work in it currently, a good chunk of that just me learning how to code, but an hour of writing might take 2-5 minutes to read and none of that counts planning time.
If you're just doing a kinetic novel, pure writing and a little sprite stuff, no mechanics, you could probably get a good amount done in 6-12 months. If you want to make a fully fledged RPG with no coding experience, much more time.