Theoretically, there is no strict deadline for long-term auctions. All long-term auctions use the Ascending Clock Algorithm.
As such, they could extend for multiple rounds if there is overdemand.
For monthly auctions, the last round of the auction takes place on the last day of the month before the runtime starts between 13:00 and 14:00 CET.
If by the end of this round no capacity is allocated, the auction is terminated and the capacity will be re-marketed in the next short-term auctions.