Typically, a client licenses whatever they want in a tune....from 1 second to the whole thing. Time used, and exclusivity of use, affect the price...along with the quality of the work and the demand for a partucular composer.
Take a TV or radio serial......they like to have a main introduction theme...and related variations to use throughout the show...and a closing theme, also related, musically. If they can get that material in one work, it saves them money. And it can make a work with a lot of variation attractive to a producer. One of the things NOT in this work is a time-change. And that's a good thing to have....I keep listening to broadcasts, and take notice of the music, and how it's used. These observations are based upon attentive listening, mostly. It's where I want to be, so I try to educate myself.
If you want to hear a great example of one tune with variation used in a broadcast, I can recommend "Truth for Life" a Christian sermon radio broadcast. The music is great, and all taken from one work with a lot of variation. Just go to the website, and click onto any half-hour sermon. Listen to the music. It's a great orchestral piece, probably MIDI. And the composer is making some serious air-time money on it. The outfit is on a tight budget....they get the most for their buck in this single work.
So...I do try to let things morph. If a client likes a particular part, he can loop it. I get credited for total time of broadcast. A 10-second loop played for a minute is a minute of creditable time for a composer!
But, also, I remind myself that I should just follow whatever muse, and create what I hear in my head. A lot of the variation in this one just came as I was assembling it. Not every turn was pre-conceived. Just happened as I went. I think if I tried to plan everything, it wouldn't be as good as what intuition and inspiration can do. But knowing that variation can be really good, it keeps my less creative impulses from ruining ideas that come from a better place.
I just make stuff....and hope that someone likes it. I hope I like it.....not always....but someone else might. There's someone I know who raved about my worst tune....the one I'm most embarrassed by.