I would say I usually spend around 7€ for an entire day. But there's a mix of common sense and also agreement with the barista. I will generally consume more or leave if the coffee shop gets really busy, or change my seat to the least used one (next to the noisy coffee machine). The baristas also think I'm good company, so that also plays in favour of me staying longer without consuming excessively. And from time to time I help them out if there's something they can't do alone.

I also found other coffee shop that are more work focused. They are usually more spacious and with big shared tables. They mind less if you don't consume regularly. Of course, that's always as long as they don't get overly busy. In general you will notice if your presence is a "problem", you will be asked if you want to consume more.

Edit: I'm located in Prague, CZ.