Can a SIGMET last less than initially forecasted?
Yes, SIGMETs can be cancelled.

A quick web search uncovered, for example, the ICAO Asia/Pacific Regional SIGMET Guide, 6th edition, May 2017, which on page 3-16 (PDF page 24) says, in part:


  3.5.4. Cancellation of SIGMET
  
  3.5.4.1. Annex 3, 7.1.2 requires that "SIGMET information shall be cancelled when the phenomena are no longer occurring or are no longer expected to occur in the area”.
  
  3.5.4.2. As such, it is mandatory for an MWO to cancel any SIGMET that is currently valid but for which the specified phenomena no longer exists or is expected to exist.


It appears that the cancellation is communicated with a new SIGMET, valid until the expiration time of the original SIGMET, specifying "CNL SIGMET XX" where XX is the original SIGMET sequence number. Sections 3.5.4.3 and 3.5.4.4 specify the format of the cancellation SIGMET. MWO is shorthand for Meteorological watch office.

SIGMETs can also be amended or corrected after issuance.

From the same document, the very next section, snipped for brevity:


  3.5.5. Amendment/correction of SIGMET
  
  3.5.5.1. If it is known that an existing SIGMET no longer accurately describes the existing or expected future evolution of the phenomena a new SIGMET, correctly describing the hazard should be issued, followed immediately by a cancellation of the original, erroneous SIGMET. [...]


So it is entirely possible that on the 31st at 11:00, the SIGMET originally issued with a validity of 310800/311200 is no longer in effect.