A Spell Scroll has the same cast time as the original spell
This is made clear, after the 2015 DMG errata, in the description for a Spell Scroll:
A spell scroll bears the words of a single spell, written as a mystical cipher. If the spell is on your class's spell list, you can read the scroll and cast its spell without having to provide any of the spell's components. Otherwise, the scroll is unintelligible. Casting the spell by reading the scroll requires the spell's normal casting time. Once the spell is cast, the words on the scroll fade, and it crumbles to dust. If the casting is interrupted, the scroll is not lost.
—Spell Scroll, Dungeon Master's Guide, pg. 200
So for Glyph of Warding, a spell that has a cast time of one hour, it would take an hour to cast a Scroll of Glyph of Warding.