[squid-users] how often should squid send If-Modified-Since: requests for a given resource?

I have squid running as an accelerator in front of a site
certain of whose pages generate Last-Modified: headers and
respond to If-Modified-Since: conditional GETs. When
viewing such a page through the accelerator, squid will
send an IMS: request to the back-end on each page view,
even if it is less than one second since the last such
request was sent.

Is this the expected behaviour? It seems a bit pointless,
given that the resolution of date in the headers is only
1s, so if the most recent version of the object squid has
cached has Last-Modified: during the current second,
there's no chance that an IMS: request is going to get a
different version of it.

(I appreciate, by the way, that this is all completely
broken anyway, because the underlying pages may change
more than one per second and this mechanism won't spot
that. But that's life.)

-- 
``My teacher's face when he worked out what I was doing was a picture. A
  picture of howling existential despair. So no change there, then.''
  (Dominic Fox, on abbreviations)