There are plenty of exceptions, there's really a distribution of times. How much does that distribution vary from one culture to another?
For example, I've been in churches where hymns are sung at half the speed they are in most white American churches, and heard singers go on for an hour (at least--I fell asleep).
I suppose with the appropriate use of keywords you might be able to use Youtube to get a distribution of the lengths of raga songs, but the numbers won't be quite clean (multiple covers, people yacking about songs, etc).
Has anybody looked at that? Is there some natural length for songs, or is it entirely cultural?