If after a year from first release, that criteria isn't met; then it'll be a difficult situation.
I don't want to discourage you, but going into (indie) game development wanting to make money is like playing lottery with the intention to hit it big. Most small businesses fail, and indie games are a small business (doesn't matter if you're a solo dev, or have 2-3-4-5-X people working alongside, or
for you). Game development is difficult - there's a glut of titles, and it's hard to stand out, regardless of genre. Adult games are even worse, because you're targeting a much smaller slice of the general population (Grandma isn't likely to buy
"Horny Titcows XXX" for her grandson's 11th birthday!). Not only that, but you're further narrowing your player base by including certain fetishes while automatically excluding most others (best example being NTR, or lack thereof).
You should develop a game because you yourself want to play it. Because it's not being made by anyone else, and you have an amazing idea you want to bring to life. Something that scratches an itch that no one else can scratch
for you. If you're going into this with the outlook that you'll abandon a project in X months unless it brings in Y money, then I'm afraid you're doing this wrong. Not because you'll most likely fail, but because your contentment hinges on factors outside your immediate control (financial success). There's nothing wrong with wanting to earn money or see your project become successful, but you shouldn't
plan one around money being the ultimate goal. In that case, you're better off doing a 6-month accounting course and getting a part-time job handling books for legit businesses, you'll earn more than 99% of game developers - adult ones includes!
What I'm trying to say is, figure out a game that you'll want to make, regardless of outside circumstances - something you can be passionate about even if it never earns a dime... something you'll want to come home to every day after work/school, and pour your free time and energy into, because you
want to, not because you're hoping to hit it big. That's the only way to keep going when life decides to throw you curve balls and takes a shit on your plans.
Just my 2 cents, for whatever it's worth.