The simple, honest answer to that question is "Now"!
If you own a restaurant, bar, pub, confectionery and you are looking for a Digital Marketing solution, then I strongly recommend that you start off with a website. This is contrary to popular belief held by most restaurateurs. Most believe that the easiest way to get on the Digital Marketing bandwagon is to get a Facebook page and create an Instagram account. Perhaps, even a Twitter account.
So, when should you start building your website? Once again, right now!
You see, like all good things a website takes time and effort. A website's popularity also depends on its age.
The longer your domain has been around, the more search engines respect it. So, all things being equal, if there are two identical restaurants with identical websites but one is older than the other by 5 years, then the older website/domain will appear higher than the newer website.
So, the more you delay creating your domain, the longer it will take your website to be respected and recognized by search engines.
Ever noticed the listing on the right side of a business when you are searching for a restaurant? That's Google listing for you. It is an incredibly powerful tool for getting customers. Given below is a snapshot which doesn't have a website. Imagine their loss of free organic traffic generated and the resultant loss of customers!
In my opinion, having a website, even if it's a low-cost website up and running as soon as possible. You can also read How to Design a Website for a Restaurant for more information.