When does the fire watch start and end for an impairment of a fire safety system?

Any examples are for illustrative purposes only.  

If there is an impairment of a fire alarm or sprinkler system (see EC.02.03.05 for related systems), the clock starts at the time of the impairment. If the system is restored within the four hours for fire alarm systems or 10 hours (cumulative) for fire sprinkler systems, the clock stops.  The time-frame noted for each system is a cumulative period of time over 24 hours rather than an individual occurrence.  In other words, if the sprinkler system is taken offline for a repair for 8 hours, then later in evening it needs to go down for additional repairs for another 3 hours, then this meets the cumulative 10 hours in a 24 hour period. 

LS.01.02.01 EP 2 requires notification and fire watch times to be documented.  Additionally, according to the appendix in NFPA 101 (2012) for 9.6.1.6, those assigned to the fire watch should be specially trained in fire prevention, in fire department notification, and understand fire safety.  Most State AHJs have specified that the person assigned to the fire watch should have no other duties and the area should be monitored consistently.  Refer to your AHJ for further guidance. 

Reference LS.01.02.01 EP 2