Well, rulebook:
"A Full Action requires a character’s complete attention to accomplish. "
"A Half Action is fairly simple; it requires some effort or concentration, but not so much that it consumes a character’s entire turn. "
Not time, but attention or concentration.
Turn described through actions, and round described through turns; also we have rough assessment for round as "something about 5 secs" ("Each round represents roughly five seconds, regardless of how many characters are involved.", if you want exact wording).