No problem. It's just a couple directives in your php.ini that control this behavior, and they're commented pretty good.
; Define the probability that the 'garbage collection' process is started
; on every session initialization.
; The probability is calculated by using gc_probability/gc_divisor,
; e.g. 1/100 means there is a 1% chance that the GC process starts
; on each request.
session.gc_probability = 1
session.gc_divisor = 1000
; After this number of seconds, stored data will be seen as 'garbage' and
; cleaned up by the garbage collection process.
session.gc_maxlifetime = 1440
So you just setup the probability of the garbage collector to run. It says when a session is initialized, but this really means when session_start is called. Then you set the seconds under maxlifetime before a session file would be considered garbage.
