in DJI Go App, one can check to see how many times battery has been charged?  Anyone know how many charges it take befrore the battery need to be thrown out?

Not sure if it applies to Spark also, but here a DJI member replied with 250 cycles.
https://forum.dji.com/thread-44343-1-1.html

250 cycles is rather short for lithium poly HV....   500 cycles for lithium poly high voltage would be my guess .. it really depends how dji doped the batteries, always trade offs, but 250 cycles seems really short.  

The warranty of Spark battery is 6 months and charge cycle should be less than 200 times. A warning message will pop up if the battery goes wrong.

I have 3 batteries with 40 charges each...

You could use something like airdata.com to parse your flight logs. It will automatically create stats for your individual batteries as well (how many times have they been charged and so on). If the Spark is powered on you can also check the number of cycles for the current battery in the battery tab.

Fly them till the flight time starts going south or the voltage is irregular.