In my Spark there is 1480 mAH LIPO battery.

What is the C-rate of this battery?

Some Lipo batteries are 35C, but some of them are 40C.
If Spark battery C-rate is 35C, then the nominal current of this battery will be 51 Amper.

The current rating stated by RC hobby batteries' manufacturers is a maximum, rather than the nominal figure. If it's always met by the particular products is another story. DJI doesn't state this figure because it's irrelevant for average user of an integrated product like Spark. Whatever the maximum current for Spark's battery is, it's clearly enough to meet the instantenous demand of every situation the drone is designed to handle. What we can estimate is the average current during the flight. If  you're running your fully charged battery down to some 15% in 12 minutes, that means your Spark was consuming an average of 6.3 A of current, give or take, averaged over the duration of the flight. There will be some current spikes during some aggressive  manouvers, flying in Sport Mode, negotiating the wind, etc. but you won't even come close to drawing anything near 50 amps from the small Spark battery (not like there was ever any need to). I guess that's my long-winded way of saying we don't really know, heh...

Thanks very much