It doesn't matter if you sang for 10 minutes or for 100 hours. There is no magic threshold number. If you identify yourself as a singer, you can claim to be one legitimately.
You would only be dishonest if you claimed to be a professional singer or a regular singer.
I don't think that claim would have any merit though, much as if someone opened up Chrome dev tools one day, edited an html attribute and solely based on that experience called themselves a web developer. I feel it would be dishonest, or at the very least woefully ignorant of common usage of language when someone is described as an *-er.
Please answer my question about me describing myself as a singer. Would you feel I was being dishonest as describing myself as such?