Forecast the remaining time of a loop

Good Evening,

I am trying to estimate the remaining time to the end of a loop; I've used:

start = datetime.now()
progress = 0

for i in range(1000): 

    #do a few calculations

    progress += 1

    stop = datetime.now()
    execution_time = stop-start 

    remaining = execution_time * ( 1000 - progress )

    print("Progress:", progress, "%, estimated", remaining, "time remaining")

But it does not seem to work properly, since it goes up to minutes, even though the loop would take 20 seconds in total, and decrease quickly when reaching the end.

How can I try to forecast the remaining time of a loop efficiently and correctly?