Depending on the age of 172, it may havea Continental or Lycoming fitted.
Lycoming O-320s have a TBO of 2000hrs.
Lycoming recommend the engine is overhauled in its 12th year regardless of hours.
Under CAA/EASA regs (CAA CAP747 Generic Requirement No.24 you can extend these periods a maximum of 20% depending on certain criteria and continue to use the aircraft for flying training and PPL hire (what used to be classed as Public Transport Category Certificate of Airworthiness).
http://www.caa.co.uk/docs/33/CAP747.PDF
This means a max hours of 2400 and calendar time of 14.4 years.
Continentals are more complicated (more parts!) therefore potentially there is more to go wrong. There are more Lycomings powering GA aircraft then any other....
Early C172s had Continental O-300 145hp 6 cylinder engines. Lycoming powered variants up to 172M had 150hp Lycomings. 172Ns had 160hp but an O-320H2AD which is best avoided. Different construction and a camshaft which doesn't tend to last too long (far worse than a standard Lyc). IIRC Continental will be 1800hr TBO (+ 20%).
172Ps were the last of the old school 172s, the newer ones are either 160hp or 180hp (SP).
There is no reason why a 172 engine won't go to 2400hrs as long as it has been maintained correctly. Best to avoid 172Ns though....
Sometimes you might find more help on GA stuff at:
http://forums.flyer.co.uk/
Good luck!