I don't have any reliable statistics, but I'm going to guess that out of 100 times that reverb is used, in 99 cases then it doesn't change from the beginning of a song to the end. Check your own work and see if I'm wrong.
But I have a great example of how changing the reverb through the duration of a song can work really well. And, what's more, in this instance it's in live performance, namely Lithuania's entry for the Eurovision Song Contest 2018 titled When We're Old. Let's skip right to the example. Listen all the way through. Or audition it at 0:13, then 0:57, then 1:20, then you will probably want to go back and listen to the whole thing.
At the beginning there seems to be no reverb at all; maybe just a hint of auditorium atmosphere. The performer, Ieva Zasimauskaite , does a great job of controlling breath blasting and mouth noises with the mic so close, which even with today's microphone technology can still be problematical. A finely-judged reverb kicks in at 0:57 which helps propel the song along, then at 1:20 there seems to be a delay effect that further adds to the density of the reverb. I say 'seems to be' because it might be pre-recorded on the backing track. (Singers are live in this show, but instruments are pre-recorded. There may also be pre-recorded background vocals but they must not replace or assist the live vocal).
It's worth thinking about the live element here because things are different to the studio. In the studio it would be possible to automate the reverb to switch it in, then change program, at appropriate points in the song. In a live performance, then the changes to the reverb would also be live. In the studio we would talk about 'automating' the reverb. But here I feel that 'animating' is a more appropriate word, similar to how the word is used in video editing.
As an aside, it would technically be feasible to automate the reverb, since the backing track is fixed and the time points of the changes are known exactly. If I wanted to to this I would probably want to have a timecode track in the audio driving a MIDI sequencer providing control or program change messages to the reverb. Or... I would record the reverb into the backing track, so none of the reverb is live. That's easier, but does it break the rules?
One further point is that if you research further into videos of the event on YouTube, you will find one that is shot, very shakily, by a member of the audience. Guess what - the reverb is there from the beginning, and it's huge. So the live audience heard something different to the broadcast audience. Interesting.
Anyway, going back to my point, this is a great example of animating reverb. It's a useful studio technique and, as we see here, works well in live performance too.
Since this article was written, a couple of interesting quotes appeared in LSi mag, from sound designer Daniel Bekerman who was Head of Sound on this production...
"...the effects for each song are pre-programmed to follow the show timecode..."
"...backing music has to be 28 tracks - 14 will be the musical tracks, then there will be tracks with voices, tracks with click track, tracks with effects."
The full interview is in the June 2018 issue of LSi.Come on the FREE COURSE TOUR