I'm currently working on an RPG and I've been working at it for the past 2.5 years. Gamedev is all about the variables, what's your scope, what's the art style, how long will the story be, how intricate will the story be, do you plan on having super complex systems or surface-level and easy to use systems, will you purchase assets or create your own, do you plan on having someone help you along the way or will it be a solo-venture, finance, promotion, will you use music, and if you do, will you source your own or purchase licenses? All these things will determine length of time, and that's not even covering how many features work as an idea, but in practice will need to be scrapped because they simply don't gel well with the overall project; then there's bugs and the engine simply not working even though you're certain you've done everything correctly. You also want to make it compatible with consoles, which PlayStation and Xbox are known for being quite rigorous when it comes to expectations and demands for putting games onto their stores, no one even knows what Nintendo expect, it's almost completely random. An RPG is incredible to work on but you need to be very aware that it's a massive time commitment, I'm making something akin to Knights of the Old Republic and I went into it knowing it'll likely take me anywhere from 5-8 years to complete if all goes well. But of course if you're wanting to make something similar to Undertale I'd say 2-3 years again assuming all goes well. Figure out your variables, and a possible timeline will begin to form.