For 50’s the maximum rest should be 20 seconds. If I learned correctly, if you take more than 20 sec rest, then your body has to start “its engine” again, and you will not be gaining any training from the repeats. It would be like you were just training for the 50.
I tell my kids (high school age) to look at the pace clock and take 20 sec rest based on when they finish instead of telling them the interval is 50 seconds. I know there is a bulletin or two that discusses how this is done. I hope that helps.