From my experience with previous unexpected maintenances, it could last for roughly the same time as the regular maintenance or even longer. If it's a tiny fix, they usually don't bother until next maint anyways.
8 hours maintance and now more bug fixing that now over 3 and half hours so much fun to come back for that and maybe it will be an arollback yayy where should i play wich server?
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xd
It can vary from case to case. Sometimes it's fixed quickly, sometimes it takes a bit longer.
Once we have an estimated time it will be up on the web post:
http://maplestory.nexon.net/news/22598/unscheduled-maintenance-may-11-2017
Deducing from Arwoo's comment, identifying the issue is what took most of the time.
dont slay me!
Look at what happened when V patch came out.