How long is a building permit valid?

Every permit issued shall become invalid unless the work on the site authorized by such permit is commenced within 180 days after its issuance, or if the work authorized by a permit is suspended or abandoned for a period of 180 days after the time the work is commenced. The building official is authorized to grant, in writing, one or more extensions of time; for period of not more than 180 days each. The extensions shall be requested in writing and justifiable cause shall be demonstrated.