Timescales are important when creating a new ecommerce website, especially when you need to meet a specific deadline. Even if you have no deadline it is still vital to get your site up and running as soon as you can so you can start to make revenue.
Some of the traps people seem to fall into is the attraction of extra modules and functionalities to make their site better. Unless you have extra modules and functionalities already agreed before the start of the project, it really isn't a good idea to keep adding more modules during the site is being built. This pushes the deadline for the site going live further back, meaning you won't start to make money as soon as you would have hoped.
Sure you want the best possible site within your budget but it's more important to get your site live and start making some money from it. Any extra modules and functionalities that you will want to add, it is best to do this AFTER the site has gone live, at least then the money you spend upgrading the site you will get back through customers spending on the site.