I suppose to some degree it depends on type. Some of the more modern flight deck setups are very quick to complete indeed - on my type, easily achievable in 10 mins or less from arriving at a 'cold' a/c to being ready for pushback/start with all performance calcs complete and departure briefing done. Usually, it's a fairly relaxed process though (especially first thing in the morning), and often takes more like 15 - 20 mins by the time all the chatting and tea-drinking is complete.
As the guys said above though, it is the cabin prep and security checks that take the longest, and then you have to contend with the whole boarding process. Would expect on an average day, minimum time from arrival to doors closed would be 25 mins for a 737 or smaller, and for the bigger Scarebuses, at least 30 mins. Again, it normally seems to take 30 - 35 mins,
From experience, charter ops seem take a little longer to prep than scheduled ops.