The regs don't say much about logging SIC time. However, that's the way the company and regs count the time towards your daily, 48 hour, 7, 30, and 90 day and 12 month limits.
In theory no one's supposed to be in the seat more than 8 hours. But I just did DWC - AUH - PVG. The DWC - AUH leg was 44 minutes block (a new personal record beating my 55 minutes MFM - HKG.) The company will figure we all flew 30 minutes that leg. Usually if I'm the third wheel for a short hop I'll try to give the other pilots an extra 15 minutes or 30 minutes rest the next leg.