yes, im so glad i asked you to be super literal lol. i meant she might not have a set time limit like i can only fly for 3 minutes and 47 and a half seconds. lol. in other words she may beable to fly for an undetermined amount of time depending on several different variables.
While Robin has undoubtedly increased her skills with Wings over the past two years, her devil fruit never was in the five or six that were capable of flying. Thus, maybe her limit is something around 30sec-1min, or possibly it has a "larger" time gap of like 3 minutes like Chopper's Rumble Balls. Either way I'm sure none of us here a assuming she can sustain flight for a long time, but rather actually use it to some degree which would be suitable and beneficial to the circumstances.