If surgery lasts longer than 4 hours, or blood loss is > 1 litre, consider a second dose. The second dose should be administered at an interval not greater than two times the half-life of the drug.

For cefazolin, cefuroxime, amoxicillin, augmentin and flucloxacillin this would be 4 hours (same as inital dose). 

For metronidazole 8 hours (same as initial dose).