It doesn't expire, and is valid until a more current one is received.
5.2 One-Minute Observations
Once each minute, ASOS performs internal diagnostic
and QC checks on sensor data and then updates the One
Minute Observation (OMO). The OMO is encoded in
METAR format and includes all basic weather parameters
found in the body of the METAR plus selected automated
remarks. The OMO also accepts manually entered remarks
(via OID keyboard entry), such as variable visibility and
tower visibility. The basic difference between the OMO
and the METAR/SPECI is that the OMO is generally not
transmitted long-line beyond the local FAA or NWS communications
network node (see bullet 3 below). OMO
information is collected during the 60-second period ending
at M+00 and made available to users each minute at
M+23 (23 seconds past the current minute) through the
following media:
OID, VDU, and authorized dial-in user computer
display screens.
42
controller are both signed-off the ASOS, the AUTO report
modifier is restored to the METAR/SPECI report.
When an aircraft mishap occurs, the on-site observer
can (and should) initiate a local special observation. In this
event, the remark “(ACFT MSHP)” is appended to the local
SPECI report, which is sent to the ASOS printer and
stored in memory for 31 days. The long-line version of the
SPECI report is sent without the (ACFT MSHP) indicator.
All ASOS reports that are transmitted through the NWS
long-line communications network are captured at the NWS
Systems Monitoring and Coordination Center (SMCC) and
compiled with other transmitted data into a daily Selective
Records Retention System (SRRS) tape. This tape is sent
to the National Climatic Data Center (NCDC) once a day
for archiving and inclusion in various climatic summaries.
NCDC also directly downloads data from
If the FAA was really going after you, simply pulling the RAPTOR data if your in a radar environment, and co aligning it with the NWS coded archive, would clear you.
If you really wanted to get technical, the Voice transmitted weather is not the actual station transcribed weather accurate to the minute. Many of the sensors require up to a 10 minute data stream to average, and perform QC checks. eg, if the Vis sensor misses 2 recordings over 10 minutes, the data will be omitted.
As long as you get the weather in a reasonable time before crossing the FAF, you are good to go. I am pretty sure that a FED would get all sorts of pissed if a crewmember went heads down, and off freq to get the instant weather as they crossed the FAF, while configuring on a good IMC approach.
Honestly, if you really want to frett the details, then how come you are only looking at the instant weather for a 10m area that is most likely not in your landing zone? How many times have you seen it call 10/clr when you can't see half the runway?