Timer needed on bathroom vent fan?
IRC - 2006 


Requires bath fans only if no operable window.
Bathroom ventilation can operate intermittently at a minimum of 50 cfm or continuously at a minimum of 20 cfm, the same as 62
Bath fans must meet the design airflow either through on airflow either through on site testing or using their certified rated flow at 0.25” water column.
Bath fans must be rated at 3.0 sones or less or be replaced by a pickup grille for a remote fan.


Also I don't think the IRC has had any major updates in this area since this 2006 publication.  So no mention at all in the IRC of timers.

However...

On the west coast especially right now it is all about energy efficiency.  They want everything on timers and dimmers.  We did two houses in Oregon last spring and the inspector mentioned the timers we had on our fans.  (I have put timers on bathroom fans for at least 10 years as my "cheap" install)  So he tells us it is code but that he has been passing some without, given they promise to upgrade to this.  He shot me a link and I will throw in a quote that should answer your question:


  Controls:
  
  The code now requires that intermitte nt fans be controlled by a
  dehumidistat, timer or similar means of automatic control. This
  requirement can be found in Section M1507.4. A light switch
  controlling both the fan and light is not considered an automatic mean
  s of control, and is no longer allowed in rooms containing a bathing
  fixture. It is speculated that the use of a 30 minute or 60 minute
  spring-wound wall switch timer will be the predominate means of
  controllin g the typical bath fan system under the 2008 code. Some
  folks are confused by the language “.....o r similar means of
  automatic control.” The intent was not to have the fan come-on
  automatically, but to shut-off automatically. When using a
  dehumidistat, the humidly percentage is set by the user, and when the
  humidly level reaches that set point the fan will turn on
  automatically and run until humidity levels dr op below the set point.
  The same applies to a spring- wound timer, the intent is to have the
  user manually set the timer to shut-off sev eral minutes after they
  have left the room. This new code language is not expected to cu re
  all moisture related problems associated with a bathing facilities
  because it still requires participation from the occupants of the
  home, but it’s a good start. Although a minimum and/or maximum
  duration for the timers was not specified in the code, it is
  recommended that a minimum 30 minute or maximum 60 minute timer be
  used. The use of longer duration timers is not recommended since there
  is no need for the fan to run for hours after the user leaves the
  area. The 30 min./60 max. numbers were discussed during the code
  review process and this was the recommendation of the code change
  committee. 
  
  What’s an example of “.....or similar means of au tomatic control”?
  One example might be a motion detector/sensor which turns on when it
  senses moveme nt in the room and stays on for a period of time after
  movement has stopped. This method is frequently used successfully to
  activ ate lights in commercial bathrooms, and there’s no reason it
  couldn’t be used for residential bath fans too. The intent of this
  language was not to limit the activation of the fan to a timer or a
  dehumidistat, but to allow for some design flexibility and allow the
  use of new technologies.


I think if I remember you are in NY.  I do most of my work in IL/MO.  I doubt the energy conservation codes have passed through in our areas yet.  But... they are coming and it is a good thing.  If you want exacts this will probably be a state by state resolution and then you have to have someone locally enforcing it.  So to answer your question is it code?  Yes and no.  Should it be standard now for anyone doing an install?  Yes.