Determining total engine life in relation to SMOH/SFOH
2000 is common but that depends on the engine. The official TBO is what ever the TC/manufacture states, here is the list for of the Lycomings published by Lycoming and here is the continental list. These are manufacturer recommended TBO's and are presumably based on data and tests they have run but they are in the end an estimate and usually reflect a relatively cautious value. 

The "actual" TBO is a somewhat subjective number that no one can give you. Internet data and general collaboration can help you get a picture of what to expect for a given airframe/engine combination but individual units must be handled on a case by case basis. When assessing any aircraft its good to look for statements like "generally makes it to TBO" or "this aircraft had a lot of engine issues" or "oh man that turbo charger made that a finicky power plant" then come to your own conclusion. Forums, message boards, and old APOA articles can be your friend on these matters. Oil analysis and compression testing is the best objective way to measure engine health and a good pre-buy is important for that. Some people say "if it ain't makin' metal keep on flying it"