Exploring much deeper into the event i found out, that it is possible to play endlessly if nobody of your citizens explores the map!
I was able to play 100 min. without triggering the event - finally a Wood cutter reached far out east for a tree and triggered the event.
So this leads to the following fact:
In the first event (build some buildings...) there should be also the building of an sentry to force the conquering of the map.
Exploring much deeper into the event i found out, that it is possible to play endlessly if nobody of your citizens explores the map!
I was able to play 100 min. without triggering the event - finally a Wood cutter reached far out east for a tree and triggered the event.
So this leads to the following fact:
In the first event (build some buildings...) there should be also the building of an sentry to force the conquering of the map.
I attached a second replay and savegame.