Timescales on projects - realistic to go solo?

Hey everyone! So I’m having great fun learning to code and getting into comp sci. (I love how the internet allows you to learn stuff) anyway, just a question based around timescales.

I just need to make sure I’m being realistic in my ambitions. So I’m interested to know about two type of applications. How long would it take for one person to build a website similar to khan academy assuming they had to build it from scratch and knew how it was all going to work before hand, and obvs all the vids etc are already made.

(This probs shows my naïveté - is this actually a thing?), and secondly, a simple multiplayer game. I have some projects in mind that I would like to create but I just wanted to ask if I am dreaming a bit if I wanted to do this all byself and see sunlight occasionally!

I guess another way of asking is if you knew what you were doing how Long it would it take! Anyways, thanks for reading! Looking forward to so some awesome replies! Rich