Figuring out how long any project is going to take is a matter of experience, which is why I would set smaller targets for sub-pieces until you're done. Goals for yourself, NOT for public sharing or announcing.
It IS entirely possible to solo projects, but you have to figure out the scale vs. time you can invest. You're not going to make a Witcher 3 by yourself within one life-time alone... but there are hundreds of really good small indie games put together my single developers or really small teams.
However you end up at an estimate for yourself though, ALWAYS "Mr. Scott" it. "Ach laddy, ya didnae tell 'im how long it'd really take?!?"
Because I do a lot of freelance work for clients on full stack solutions (back end, front end, from scratch) I ALWAYS Mr. Scott my estimates. That means I figure out how long it would REALLY take me, then I DOUBLE IT when I tell the client.
It is ALWAYS better to deliver ahead of time than to miss a deadline, particularly when you have no clue what might go bits-up face-down along the way. You have to give yourself that fudge factor as I've seen far too many people make their time estimates on the assumption that everything will go right on the first try... Give yourself some leeway for if something goes awry!