How soon does a clinician need to document the service after performing the service?

If you are asking this question, it is probably because a physician or other clinician in your practice is behind in documenting their encounters.  

Here is what CMS says in the Claims Processing: (Publication 100-04, Chapter 12, Section 30.6.1 A

The service should be documented during, or as soon as practicable after it is provided in order to maintain an accurate medical record.

That's it.  CMS does not give a time limit of hours or days.   There are hospital rules about documenting the physician component of medical records, and Health Information departments keep track of those and remind physicians who fall behind.

What policies are reasonable in a phyisician office, and how should those be enforced?  Many groups develop a policy that all documentation must be completed within three days. A week at the longest.  After that time, how good is the clinician's memory?  Groups that use electronic medical records are able to monitor completion of medical records through the EMR.  Enforcement is dictated by the governance stucture.  Sometimes, the Board of a private medical practice will have regulations about chart completion.  If a hospital employs the physician group, there may be requirements in the Compliance Plan, by-laws, or employment agreement.  When a clinician falls behind, the group must address it, using whatever governance structure is available, quickly and definitively.