Setting the CacheControlMode parameter as described in IIS7 documentation should do what you want:
To set the expires response header to expire content periodically and
to set the number of seconds, minutes, hours, or days at which to
expire the content, use the following syntax:
appcmd set config /section:staticContent /clientCache.cacheControlMode:UseMaxAge /clientCache.cacheControlMaxAge: timeSpan
The variable timeSpan is the time at which to expire the content. The
format for timeSpan is d.hh:mm:ss, where d is the optional number of
days, hh is the number of hours, mm is the number of minutes, and ss
is the number of seconds at which to expire the content. For example,
to set the cache to expire every two days, type the following and then
press ENTER:
appcmd set config /section:staticContent /clientCache.cacheControlMode:UseMaxAge /clientCache.cacheControlMaxAge:2.00:00:00