Just once, humming (not particularly well), not singing.

The song in question is sung with a very clear voice, with minimal instrumental backing. That may suggest the limiting factor is Google's ability to parse songs, rather than users' ability to hum them.

Why not try testing it for yourself? That way, you'll have a definitive set of results to look at that you can guarantee.

That wouldn't answer the question. Searching a song that you remember from the past is nothing like listening to a song on YouTube and then immediately trying to sing it back to Google.

That's a fair point. Thanks for providing an explanation!