You can only fight in Zenith 8 times total. However, you are given the option to fight 4 times in one day. It is up to you to allot your fights during those 4 days.

You are only expected to fight on average 2 times a day for those 4 days and if you go above the average on one day, you can go easy another day. The time is really rough, but it should not be too difficult to clear 2 fights a day.

Zenith Arena requires a very small time gap because it has perfect matchmaking. The way the event is structured requires that as many people are queued up at the same time.

If the event time is extended by several hours, matchmaking will be extended exponentially because the game will wait as long as it takes to get an opponent with the same exact wins as you.