session.cache_expire specifies time-to-live for cached session pages in minutes, this has no effect for nocache limiter. Defaults to 180. See also session_cache_expire().
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Note: Setting new_cache_expire is of value only, if session.cache_limiter is set to a value different from nocache.
By default the session.cache_limiter is set to `nocache`, so you don't have to worry about the 180 minute expiration.
Linky contains a pretty good user-contributed description of cache use.