How fast is the update cycle for the 0-20 mA Output signal?

How fast is the update cycle for the 0-20 mA output signal? Is it safe enough to check the 0-20 mA signal with a PLC every second in order to receive a peak?

The Analog output in your Hardy controller will update the current reading for each update of your unit.  Analog out is an inheritably slow compared to a digital output.  There is a resistance to change across the ma signal due to internal electronics.  That speed of change is not covered in this article.

Update rate for each of the Hardy controllers.
For the HI 2151/30WC, HI 2110 WI, HI 2160 RC and the older HI 2151/20WC, this is at 20 times per second (50 ms).

For the HI 3010, HI 3030, HI 6310, and the HI 3600, this is at 55 times per second (18 ms).

For the HI 4050 this is at 100 times per second (10 ms)

If you were to monitor this and take readings once per second you could be missing most of the readings. Whether this would work for your application would depend on how fast the weight was being changed and if the amount of change during the one second is critical to your operation.