This is a great article by Dan around the best approach to scoping a website build (or anything for that matter). Breaking it down logically into chunks allows us to see the components, and down further and further until you can see if you’re able to get it done within the time (if not, increase your time or decrease the scope).
In my experience the general rule for the time it takes to delivery anything is the time quoted/defined plus 5%. Why? Well because if something is done early I always find a way to use the extra time to do something even better… which takes a smidge longer. Client is always happy though 😉