There seems to be a lot of information about how elections are run, but none about how often they are held.
Are elections a regular thing or are they held more ad hoc? What is the usual period between elections?
Moderator elections are held for graduated sites when the SE team feels that additional moderators are needed.
When do elections take place?
The community team periodically looks at the work load on graduated sites and will reach out to the moderation team if it looks like more help might be needed. Broadly, it's up to the moderation teams to indicate whether they need additional hands, and how many sets of hands would be ideal.
Although we'll reach out to moderators on the anniversary of their last election, just to see how they're doing, elections do not always take place at the same time every year. Sometimes a year can even go by without any election at all. Most elections are scheduled as a result of moderators reaching out to Stack Exchange to ask for additional help.