Interesting question - It's difficult to say. I can't answer for your circumstance, but it took me just over 2 years, working on and off in my spare time, and a lot more seriously over the last year. If I did it again with what I know now, the answer would change.
But also, don't let size cloud your judgement - You can code physics and build out a playable world with some enemies in 1-3 months... Maybe even a week or a weekend with some game jam experience.
However, detailing it with funky graphics, making a soundtrack, sound effects, writing dialogue, making cutscenes for multiple endings, making satisfying boss fights, making a menu system with settings and file management incl. an extras menu with achievements, making alternate game modes, secrets, actioning lists of suggestions from playtesting feedback, learning how to do things you haven't done before (like writing a shader for brightness/contrast), unexpected time required to mentally adjust between different types of tasks, getting your game to a place that you genuinely think is worth the money you're asking for, etc... Well, that all takes time. It really depends what you want to put into your game of this size. If you haven't made a game before, let's not forget failing projects until your scope aligns with your capabilities, as well! But, the more experience you have, the more the time will shrink. I constantly underestimated how long things would take with Rogue Sentry, though.
I hope this provides a little bit of insight. :)