There isn't an overall time that can be used, but you can use this http://stats.roleplay.co.uk/
Put your name in, select your name, so since you are NHS it will probably say something like STU. <Name> or what ever rank you are. It will then say 'Medic' in Orange next to it. Click it, go to the bottom where it says Logins, and then you can see how many hours you have used by using a calculator. There is a bug that says you have only logged onto Server 3. Just ignore that unless you have logged on S3.