.... just to add to my thoughts into the memorization process.....
Music, in general, is repetitious. It's the nature of the beast. As Steve mentions, verse 2 is 99.9% going to be musically the same as verse 1.
Music in itself is written in sections.
Measures. 4 beats per measure. 8 Beats per measure, .etc., etc., though not always in even counts. Some may be odd, like 3/4 or 6/8. However, the accumulated measures are a
stanza. Though measures may be different musically, in most cases there will be a
coda at the end of an certain number of measures, arbitrary and completely at the whim of the author of the music, which indicates to the musician a return to the beginning of the stanza and repeat. That complete set of measures would then become a
verse if lyrics are written and a stanza whether lyrics are written or not. (and I'm digging way back into my formative piano years here)
As with everything you learn in life, it is learned by repetition. The more you do something, the less you think about doing it and the more it becomes an extension of your automatic processes.
A beginning guitarist will most likely choose simple songs to learn. Such a Twinkle, Twinkle Little Star.
And right now let me express my sentiments to beginning guitarist. It is OK to learn Twinkle, Twinkle Little Star. It will serve you well. But as a beginning guitarist, pianist, or bassist, or trombonist, you will only learn and memorize it by repeatedly practicing the melody. Repetition.
As far as how long it takes to learn and memorize? Well, that is completely dependent upon the individual learning the tune. I may learn a particular tune faster than someone else. Likewise, they may learn a different tune faster than I.
I would not get hung up how long it takes to learn a tune. Every single player learns at a different pace. Heck, I've been learning some tunes for 25 years and still can't get them down.
Some tunes I can hear once or twice and can tell you exactly whats going on and play them without a hitch.
Learn the tunes you want and take the time you need to learn them.
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LC