These answers are unhelpful and inaccurate.
Although many great musicians do find it easy to quickly play songs by ear, there are many more who found it difficult and had to practice before they could do it quickly.
For most musicians the two big factors in how long it takes to play a song by ear are:
- How complex the song is.
A simple 3-chord song played on solo guitar will be easier to figure out by ear than a full band arrangement of an avant-garde jazz piece!
- How practiced the musician is