Time depends on your requirements and how much time you are giving. To analize time devide your website, app or software into small pieces. Then decide time of each part. Finally you will get your Actual working hours.
Oh, yeah. I create many apps alone. But it depends on whether you have a good time, and if you have the necessary skills. Otherwise, you need to seek help, both on YouTube or Google and with other people who know what you need
Yes, it depends only on how much time you want to give. If it is a serious project(except website development) it will become a full-time job for half year to some years.
If it is like a hobby, I think it can't be a very big project.