I did this for two years. The key is to switch it up. I'm lucky enough that there are many different coffee shops in my area that I can go to. The general rule of thumb, if you're planning on spending multiple hours in a single place, is to buy a little something each 1 to 2 hours. I've seen many, many people buy a single tall coffee, nothing else, and then sit in the coffee shop for 4+ hours. These people are jerks.