Its been a while but if my memory serves me w3tc has a garbage collector timer and a browser cache timer (assuming you have Browser Cache enabled). They work hand-in-hand. The garbage collector timer found under Page Cache settings is a wp-cron timer that gets triggered to check to see which cache files are old and set them as such, getting them ready for recaching.
These files are considered old based on what the Browser Cache timer is set to (under Browser Cache settings’ Expire Heading Lifetime for HTML, CSS/JS, Media files).
When files are considered old (based on the Browser Cache header lifetime) then its up to the garbage collector to eventually be run and mark them as old. If it doesnt then these browser cache expired pages will still be considered fresh.
In short, for actively changing content you might want to set your Garbage collector time low, along with the header lifetime. But be warned if you’re not careful it could constantly be in a state of recaching — slowing things down. Try not to have the Lifetime timer less than the garbage collector timer. If you do this would make your entire site’s cache become old each cycle. Unless that is what you want.
