Eventually you would hit a point where a 2 second concentric would be impossible because you are fatigued and not producing enough force. Let's say you notice that it took 3 seconds to push the weight up. Then define that as your failure point and don't go past that.
James 1:16-17 ESV
Do not be deceived, my beloved brothers. Every good gift and every perfect gift is from above, coming down from the Father of lights
With God's help...Mens sana in corpore sano