How long is a building permit valid?

2018 International Residential Code, R105.5 Expiration. Every permit issued shall become invalid unless the work authorized by such permit is commenced with 180 days after its issuance, or after commencement of work if more than 180 days pass between inspections.  The building official is authorized to grant, in writing, one or more extensions of time, for periods of 180 days each. The extension shall be requested in writing and justifiable cause demonstrated.