What are the duration of TAFs?
First you will need to learn to decode the TAF's validity period. 

TAF YMAY 022230Z 0300/0312 35010KT CAVOK
FM030800 31018KT 9999 SHRA BKN025 OVC100
INTER 0308/0312 31020G40KT 3000 +TSRA BKN010 SCT040CB
RMK FM030600 MOD TURB BLW 5000FT
T 23 24 28 33 Q 1012 1013 1014 1009

The validity of this TAF (in bold) is formatted as ddhh/ddhh. 


  Validity period of TAF is from 0000 to 1200, on the 3rd day of the
  month UTC


From here we can see that it's valid for 12 hours. 

Another example: 

TAF COR YMLT 212240Z 2200/2218 31015G28KT 6000 -RA BKN010 OVC100
TEMPO 2209/2218 2000 +TSRA BKN005 SCT040CB
RMK
T 25 21 18 15 Q 1014 1013 1013 1011


  Validity period of TAF is from 0000 until 1800 on the 22nd of the month UTC


This one is valid for 18 hours. 

You may check your country's AIP for how often they are updated. 

For instance, in Australia, GEN 3.5 - 5 describes the routine TAF service based on airport size.  



A International:  Issued 6 hourly, valid for 24 or 30 hours.
Commencement times 00, 06, 12 and 18 UTC.



B Large: Passengers > 150 000 or Movements > 75,000 
Issued 6 hourly, valid for 12 or 18 hours. Commencement times 00, 06, 12 and 18 UTC.



C Medium:
Passengers > 50,000 or Movements > 10,000
Issued 6 hourly, typically valid for 12 hours.
Commencement times are 02, 08, 14 and/or 20 UTC, except in Western Australia where commencement times are 04, 10, 16 and/or 22 UTC.



D Small:
Aerodromes meeting passenger and movement thresholds, or other operational criteria.
Issued 6 or 12 hourly, valid for up to 12 hours.
Commencement times are typically 20 and/or 02 UTC, except in Western Australia where commencement times are typically 22 and/or 04 UTC.