10-20%
The big key to getting decent service here is being recognized as a regular customer.
When I was new in-country a local explained to me that it's the opposite of the USA. In America, we think, you give me good service first, and then I'll give you a nice tip. But she said if you are new in a neighborhood, and you plan on patronizing a restaurant regularly, you may have to leave a good tip the first few times, in spite of having gotten crappy service. Then, when they get to know you and that you will tip well, then you will get good service. It seems backward, but in my experience it does work.
Another thing that works against us is that going out to eat alone is less common here. So if you do come in alone, you are regarded with suspicion, like, "well, nobody sane would go out to eat alone if they had a friend, so this person doesn't have any friends, so they must be a complete asshole..."