With DS1 I pushed "officially" for under an hour, but in reality it was more like 3 as my midwife let me push for ages before declaring me fully, hence buying me more time before the docs would want involved. I was really happy she did this because I did not want an instrumental delivery but it was really tiring and left me with bad SPD pain for months.

Fast forward 15 months later and a lovely waterbirth with my 2nd son, 15 mins 2nd stage and my SPD (which had been terrible in the pregnancy) disappeared altogether.

I would be wary of stipulating a short time such as 20 mins for 2nd stage though. Obviously I do not know your medical history, but if it is pelvis instability you are worried about then looking at alternative positions for 2nd stage such as all fours might be a better way of ensuring success with a VB.

Good luck

T
xx