We purchase template according to our requirement and than install it on latest PrestaShop version. it take arround 15-20 hour because it was my first PrestaShop project. then i add 6-8 category and around 20 product over there and other final touch to design according to my requirement and make it suitable with my logo color, take other 10-12 hours.
so it depends on how you design shop using theme or going to make by your self.also depends on number of products.
Thanks.