How long do you've to ask for the same dish before?

I think you have to wait for the server to say, “The usual?”. That’s when you will know it’s been long enough.

I disagree, with @janbb. ;-)
2 times.
First visit, start immediately with “the usual, please”.
They’ll not know what dish you want, but you’ll tell them.
Second time, ask for “the usual”.
You’ll get the usual.
We’re assuming here that you see the same waiter/waitress.

@rebbel And that the server has a good memory!

That’s built in in my tactic; the waiter will remember because it was an out of the ordinary event (the first visit).

I used to go to a certain restaurant a couple of times per week with a coworker. This is a friendly area so it is not unusual to talk with the server for few seconds.
I ordered something and the very next time I went in she asked if I wanted that same thing. I was so impressed I said “yes”.
From that day forward I was “cursed” with having to eat the same thing. If I wanted to change it I needed to make an excuse like I was having it for dinner or had leftovers.
Of course I placed this stupid burden upon my own shoulders. I’m sure she would have had no problem if I ordered something else, but I didn’t want to offend and, to be honest, I kind of liked the idea of her taking the initiative.

If you have the same waiter and order the same thing a couple of times, you can add in “I’ll have my usual, coffee black and a white western omelet with cheddar.” The waiter might take note that you are a “usual” type of customer.

I wouldn’t expect a waiter to remember, but it’s nice when they do. I think waiters have good memories generally speaking; it makes the job much harder without that skill.

In my experience, while many servers have remarkable enough memory, some don’t, so not making sure a server acknowledged every detail of an order with any detail in it, risks not getting what you want. Of course, it’s a lot easier if your “usual” is a typical undetailed thing, than if you have specific details (e.g. egg over hard, no pickle, no horseradish) that are important to you.

I get more wrong orders than I want, even when I specify everything.

@rebbel, do you actually do that? Have you ever?

@Jeruba I haven’t, but the next time I go to eat out I will.
To try to see if my tactic works!

Twice.

Thanks guys for your input.

In my experience, it was when the server said hello to me by name.

We had a local coffee shop that served breakfast and lunch, and had a lot of regulars. It was also “the place” for special occasion breakfasts like birthdays and Father’s Day (not nearly fancy enough for Mother’s Day). My kids were very impressed when the server said, “Peter, are you having your regular poached eggs?”