My guess is 'how long' depends on 'when'. If your coffee house is going to get crowded at lunchtime, neither the proprietor or queuing customers are going to love you for hogging a table. Especially if there are not enough seats to go around.
During slack times, though, what possible harm can your presence do? In fact, in a way, you are doing the coffee shop a favour; you're making it look busier than it actually is!
Having said all that, a couple of years ago, I came across an item about the interior design of American-style coffee shops. (This was in Britain.) Apparently, the numbers of comfy chairs were going to be cut back because they encouraged people to linger. So, out went Friends-inspired sofas, and in came hard dining chairs and stools. This was particularly important to businesses in central city locations where passenger turnover wasn't fast enough and that, therefore, weren't making enough profit...
Chris
