When did composers begin to write full-length ballets with extended plots?

19th century

Explanation:

Swan Lake is one of the most striking pieces of the world's classical ballet. Moscow's Bolshoi Theater premiered the show in 1877, with choreography by Julius Reisinger from a composition commissioned by Tchaikovsky - author of other works of impact on ballet history, such as The Nutcracker (1892).

One of the reasons for such success is the ability of the assembly to articulate Tchaikovsky's fantastic melody with dance and performance by dancers to an exceptional level of excellence.

Certainly, the emotional density of the show's plot, uniting feelings of hope, passion and despair, so common to human life, is one of the foundations for the success of "Swan Lake."

Remember that this ballet also demands greatly from musicians, especially violin soloists, and brings one of the greatest dramatic charges in the history of symphonic music worldwide.

It was in the 19th century in I believe 1877 when Swan Lake was commissioned. I am not sure though.