There are no records or mentions of GA meeting every five years. When a boss of bosses was selected it was supposedly for life. I believe the GA met when there was a need to elect a new boss of bosses and when there was a need to address major problems that needed to be resolved. One example is the castellammarese war when a "mini commission" was created, consisting of Nick Gentile, NJ´s Vincenzo Troia, Chicago´s Toto Lo Verde and other "men of honor" in order to bring peace between Masseria and Maranzano. This "mini commission" (which should not be confused with the Commission created in 1931) acted as messengers between the warring factions and as mediators in the dispute.
The GA dissolved in 1931 when the "Big" Commission was created. There was no need for the GA anymore because all the other smaller Families were represented by one of the bosses at the Commission.

The Commission (the one created in 1931) held original scheduled meetings every five years. The five year rule was established so that the chairs on the Commission could be ratified for a five year term. But the Commission also met to discuss issues and resolve problems in between these five-year-term-scheduled meetings.
For example, the 1956 scheduled Commission meeting in upstate New York was followed up by a 1957 meeting in Livinston, NJ when a decision was made to name a successor to Anastasia. The scheduled Commission meeting in 1961, was follwed up by meetings in 1963 to condemn Magliocco and another one, also in 1963, to discuss the Bonanno issue.