Once, as a free action; either when the creature meets for the first time or when the player declares that he would like to write a check.
A character could not try again just because more of the same creature appeared, either during the same encounter or later that week. If the initial knowledge check determines that the cleric does not know what the undead creature is _______, seeing more of them will not help. In general, if we try knowledge checks, it is only allowed if the character gains another rank or obtains new information that could influence the verification. For example, a ranger fails his check to identify a dryad; After tracing the dryad for a couple of hours, the ranger sees his tree walking towards a particularly large oak tree and places his hand on it fondly. In this case, the DM could allow a new verification, which could represent the ranger who remembers the connection between dryads and oaks.