"Reasonable" is a subjective term. There are a lot of possible things that could be going on in the life of a developer. Some make these types of games in their spare time. Others do this full-time. Some have fairly outdated machines that struggle to handle rendering images, and when they
do render them, they might not be as high in quality. Others have excellent setups and can create extremely high quality and realistic animations. There are also varying talent levels, where some developers are new to creating erotica and/or using specific software and others just seem to click with it all naturally and are amazingly gifted at doing this type of stuff. (I think they
all are talented, but the degree of talent differs from one to another.)
I'm not a developer, so you may wish to take what I say below with some degree of skepticism. But I think the following few things are good for consideration, even if you don't know the first thing about developing a game. First, you should understand that there are a lot of aspects of development, all of which will need to be managed when used. Just off the top of my head, you (or your group, if you have a team) will need to manage: Art, programming, game design (UI, mechanics, etc.), story planning, writing (esp. dialogue), music / sound effects (optional), editing, proofreading, and play testing.
You might consider looking over the course of your life for the last year, identify how busy you typically are, and go with an
average time. Another option might be to look at your
busiest month of the year, and promise that as the minimum. It really all depends on what you believe you can handle and how much you want to focus on your project during any free time you might have. I would also suggest that you give yourself some leeway: include extra time (hours per week) for creating, above and beyond the actual time it might take you. This gives you some wiggle room if you run into errors (in rendering, in programming, with unforeseen story adjustments, with translations, etc.).
Of course, an actual developer —
especially of this type of game — can give you a much more useful response than I.
Edit: I have added the following, per the recommendations of BogoHobo and Walter Victor. (I can't believe I didn't include testing: I actually do it for a living for a dot-com company. )