You have to make sure that by the time the Nationality Office receive your application, your definite leave to remain has been exactly 1 year, otherwise they will return your application. All application vary on the amount of work they have at the time, on your circumstances, how quickly the checks are passed etc. You should allow at least 3 months, but you could always call the Nationality Advice Line and they can update you on your application. With regards to referees, both have to hold british passport and know you for at least 3 years. One can be a friend, but the other must be a professional. If you have a close friend who is a professional, then ask them to complete the form for you, would be better than a GP who you only rarely see. Hope this helps.