There isn't a fixed figure. Each engine type has its own TBO (to be overhauled). This may be different even for the same engine: if it is turbo charged, approved for aerobatics etc. Where an engine has an approved extension programme this figure is commonly 20%. In the case of a 2000 TBO engine it may therefore be extended "on condition" for a further 400 hours. If the engine is used wholly for private use the on condition programme can be without limitation of extensions.