How long should I slow roast a large (7.5lb/3.5kg) chicken for?
That's the largest chicken I've ever heard of.

That said, 3 to 3.5 hours at 325℉ should be sufficient. Unfortunately time doesn't really matter when cooking chicken, but temperature. You should definitely be using a meat thermometer and cooking long enough for the breasts to reach 160℉ and the thighs 170℉. 

If you don't have a thermometer the next best thing would be to poke the thigh when you think it's done and confirm that the juices run clear. If they are not clear, then it needs to be cooked longer.