By default, Nimble Streamer sets Cache-Control header for playlists and chunks to "no-cache".
To change this behavior, use playlist_cache_control and chunk_cache_control parameters and set proper value for it in Nimble Streamer configuration file.
E.g.
playlist_cache_control = max-age=30
chunk_cache_control = max-age=120
will set 30 seconds as maximum age for playlists cache and 120 seconds for chunks.
Don't forget to restart Nimble Streamer instance after changing the value.
Read this article to learn how to control Nimble behavior through config files: http://blog.wmspanel.com/p/nimble-streamer-configuration.html