A Regency Council, often shortened to Regency, is a group of persons selected to act as a temporary government because the ruler is a minor, not present, or debilitated (wikipedia). In the game, the ruler being a minor is by far the most usual reason for a regency. In some instances, it might be caused by the complete absence of an heir. While the event text for events like Temporary Insanity of Monarch does hint at a regency because of debilitation, this doesn't actually change your ruler.
Until the heir apparent comes of age, the council reigns, with the following limitations:
- The regent cannot declare war
- The regent cannot become Holy Roman Emperor
- The regent cannot inherit or become the senior partner in a Personal Union with another country
- The regent cannot convert the Head of State into a general
- The regent cannot be excommunicated
Heir to the Throne
In Heir to the Throne, an heir is considered fit to rule at the tender age of 15. So if a king dies when his heir is only 5 years old, you know you will spend at least 10 years in a Regency, possibly more if the heir dies prematurely.
An interregnum can be regarded as a more serious form of a regency. Instead of an heir who is too young to take the throne, there isn't an heir apparent at all. It only occurs in HTTT.
In the versions of the game before 'Heir to the Throne', the current heir and his age are not indicated. You therefore have no way of knowing how long a regency will last. It is random, but it usually lasts a few years up to a couple of decades.