I would disagree with the length of time it takes comment. Some songs like "The First Time Ever I Saw Your Face" were written in 7 minutes. Cole Porter said "All the motivation I need, is the call from the producer". But people with day jobs have to make time. And some really good songs build over weeks, in my case a verse a week. Thoug I have written them in an hour. If the song is hard work and looks like it, it may be better to not sing it in public, but regard it as practice. Some people have half a song and come back to it when they have more info. A historical song needs careful checking unless you want a pedant in the audience to ruin it. That takes time. The only time I would advocate giving up after 30 minutes is if there is no good hook line or opening stanza, becaue it would signify the muse is not flowing. However if all you have after an hour is maybe two or three lines, put them in the archive (allocate a whole page for that proto-song) and revist maybe in a week, a month, a year etc. Then maybe there is a spark.

Remember the Golden Rule - and who would argue with George Bernard Shaw?