@glyn,
I've had tonsillitis many times, and if I'm too tired or a little bit sick, that's where it always starts: the tonsils. I haven't heard of your antibiotic but I take it your doctor has found an infection. From the sounds of things (your pain and difficulty swallowing), that seems right.
Duration varies but you need to take care of yourself. As in, do whatever you can to get foods in you so that you eat right, drink a lot of fluids and keep yourself warm but not hot. Also, get enough rest and don't try to do much of anything. Your body needs to recover from this, so giving it other things to do (like work at home or whatever) is generally a bad idea and can make it take even longer for you to get better. If you absolutely have to go to work or school, do so, but then get back home, get homework done (if it applies) and then the same: food, drink and rest, making sure your body temperature stays fairly consistent. Don't try to sweat it out and don't freeze. If you're cold, put on a sweatshirt no matter what the temperature is outside -- you don't want your body to have to expend too much energy on temperature controls on top of everything else.
Feel better soon.