The more we are able to remember, the better. Students usually start with the practice and finish after a couple of minutes. When we keep up with the practice and combine it with vigorous self-observation during the day, the amount of details and scenes will increase and the practice will take longer.
There is no rule of thumb for the duration, but it is beneficial to do it as thorough as we can. This will train the internal senses and also prepare the soul for the process of death. Remember that in the Tibetan Book of the Dead it says, that after death the soul of the deceased one reviews their entire life .............

Thanks:)

So it's really about the matter of amount of time we have incase to practice?

It's about the amount of details and clarity that you are able to build up. This trains your consciousness and the consciousness resides beyond time.