@Krishna Sreenivas
there is no default value. Empty value (null) means - that first-time records (which aren't detected as dups yet) will stay in a cache forever (if you have cache service with persistence directory defined, otherwise - till restart).
If you define, for example, 1 min, that means, that during 1 min, all dups will be detected, but after that time, another record won't be detected as a dup, as it will be aged off (and removed) from cache.