Typical Streaming cache length when entering "no service" on mobile device

[Johnospoton]

When you are streaming spotify radio over your mobile data while driving and travel into "no signal" area, how long of a cache is built up? I intentionally headed deeper into a T-Mobile dead spot and was expecting the song to stop within a minute or two, but 15 minutes after the one dot changed to "no service," the stream was still going strong. I wasnt using off-line content and the device didn't indicate that I was using any other provider's network. Is it switching to another providers tower or is the cached stream really that long?

iPad w/ t-mobile SIM card