Transition of Power

Do not allow a leader to serve more than once.

— The Twin Pillars

Execution

Under ordinary conditions, the Leadership Challenge is held every seven years during a window of time between Last Sunset and the start of Migration. A successful completion allows the contender to stand before the Droghi Council and be proclaimed Leader of the Stenza. The results of their hunt are ritualistically reviewed and compared to other points in their record as applicable, before the contender is declared to have succeeded.
There are exceptions, however.

Forfeit

Forfeit of the challenge is arguably rarer than cheating, although no less controversial. The first forfeit in centuries was Saeh'han's, who returned with the target human under his care and had even begun training her to live as a Stenza (insofar as possible). Because there was still time, T'zu-Crae could also challenge that year, and was declared Leader as a result. To say this sequence of events was politically and socially divisive is something of an understatement.

Cheating

During the review period, a contender may be found to have used weapons or called on assistance against the rules of the challenge. This is an automatic disqualification. (Supposing Tzim-Sha's recall circuit worked correctly, he would have received medical treatment for the DNA Bombs and then promptly been disqualified due to evidence of their use, although rumors are swirling that he was trying to "run out the clock", see below.)

"Running Out the Clock"

This is a political method employed by factions of Stenza in an effort to force a one-year period of leaderlessness, which occurs if no one successfully completes the challenge by the deadline set (traditionally the start of Migration). Because there are no rules against interfering negatively with someone's challenge attempt (i.e. helping the target human escape or otherwise making the challenge arbitrarily more difficult), this delaying tactic sees a lot of use, and some methods are considered more legitimate than others. For example, forcing someone else to challenge by forfeiting yourself (see above) is more socially acceptable than tampering with a contender's recall circuit (which risks getting that person killed). Taking a noteworthy but not overtly suspicious amount of time on the challenge is especially common.

Leaderlessness

Should the window of time close without any successful completion of the Leadership Challenge, the Stenza wait until the next calendar year before anyone is allowed to make an attempt. The cultural connotations of such a period are mixed; it can be both an inherently uncertain time vaguely reminiscent of the pre-Unification past, and also a time to reflect. It's also a time for the faction that successfully brought about this period to make their case as to why they did it, and for that case to be reviewed by both the Droghi Council and the general public. Often, to facilitate this, most external operations grind to a halt.