There are so many different ways to build a website. You might use a theme, you might use a template, or you might build it from scratch, hard coding elements to make sure it is going to work just as you want it to. The way in which you choose to build your website will really affect how long this process takes. If you have a simple, one page website that is already in template form, this will of course be a far quicker process than it would be for you to have a new website built from scratch using no theme at all.
If you are having a website designed, ask your design agency for a realistic expectation of timings. However, do be aware that these things are hard to predict. Technology can be difficult, and if things go wrong, it can take a while to fix. It isn’t worth rushing the process – take your time and get a website built that you love.