I've taken melatonin every day since mid 2018 and if anything, my sleep has gotten even better with time and I have more energy the following day. What I noticed is that if I don't take it at my usual time I get super sleepy around that time anyway.
I actually wonder if melatonin supplementation trains the body that there's a certain amount of melatonin in the system at a specific time, and when it doesn't come the body just produces it itself? It's a far shot, maybe people who supplemented it for a longer period and then stopped could confirm difficulties sleeping when they discontinued melatonin?
There were a few occasions when I didn't take melatonin, for example, I had a short night due to a plane journey the following day, or I simply forgot, and I didn't have trouble falling asleep. I haven't skipped melatonin for a few consecutive nights so it's hard to say if I'd notice an issue, but to make sure I don't have to worry about it I have a supply for a couple fo years ahead