For only a very short time — the adverb of 'fleeting'. You use it about actions that happened and were over almost at once: he smiled fleetingly, she glanced fleetingly at the door, the thought crossed her mind fleetingly. Like 'fleeting', it carries the sense that the moment was too short, that it slipped past. It very often appears with verbs of seeing, thinking and feeling — glance, wonder, smile, appear, consider — because those are the actions that come and go in an instant. It is a written, somewhat literary word; in conversation people say 'for a second', 'just for a moment' or 'quickly'. Position is flexible: 'he fleetingly wondered' and 'he wondered fleetingly' are both fine, though the second sounds more natural.