It depends of the quality you choose for your downloaded songs, as well as the number of songs in the playlist. (usually it takes about 4-5 min on 4G to download a playlist with about 200-300 songs on normal quality with an average song time 4-5 min).
You can change the quality in Settings > Music Quality > Download. Higher the quality, slower the download, higher the space it takes on your device.
Once you start to download you'll see the progress in your notifications panel. It'll show how many of the songs are downloaded, eg. 50 of 200 songs.
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