The minute you enable the sync process it will start pulling the info, for that amount of users it shouldn't take long.
Also, even if you disable it the users won't be deleted as there are some time consideration about when users will be deleted by the garbage mechanism, if you're interested the whole explanation is under LDAP chapter from SRND.
As mentioned, there is no way to test this, other than enabling it.
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HTH
java
if this helps, please rate