after tinkering for a decade, i can say 6 weeks for an 8 hr game with all custom everything. and that's crazy fast. if you're not trying to give yourself a heart attack/stroke, then on average "fast" is 2 years if your title not rotting in dev hell, and you're designing *all* the things yourself (given the skill). I also know the ins/outs of the engine, and i use templates and randomizers for everything (graphics, music). I also have a master SRD (system reference document) for my games so i don't have to think that hard about classes, values, etc. lastly, i have a group of beta testers that will tell me what i over looked. and i have a team of debuggers to fine tune the spaghetti code that's cryptically commented on.
i know not everyone has a stable of coder monkeys at home and are single people so they'll be at their passion project for years (like i am lol). also not everyone wears all the hats like i do nor have the capabilities of automating their hats (i had to, as my health was suffering).
now that i'm retired from my other jobs, and i basically have nothing else to do, i can fully devote time to it. here's to (finally!) a timely release!