System uptime is general topic which you have to write in SRS. You can write following things in System uptime.

• Developer warrants that the system shall be functionally operational within the specifications.
  b. Any time that the system is not functionally operational due to hardware failure or maintenance, server or other causes beyond Developer’s design or control shall be excluded from this.
• In the event of a failure of any program, Company shall notify Developer by telephone immediately. Upon notification of the failure, Developer may begin immediate repairs of said failures.
• After completion of installation & operation in full phase, service & maintenance will be on call basis. Mode of service will be determined by the requirement (i.e: physical presence, over phone, online service via team viewer within 24 hrs.)
• Any type of problem arises due to Company’s IT infrastructure setting (i.e: connection, server down, ISP bandwidth problem etc which is not directly involves with this software or its architecture) will be liability of the Company as it is their setup.