What happens if I hold a piece of paper horizontally with two hands and I let go of one hand?
user
If you hold a piece of paper horizontally with two hands and let go of one hand, the paper will start to tilt towards the hand that let go. The amount of tilting depends on the angle at which the paper was held and the force exerted by the remaining hand.

If the paper is held at a relatively flat angle, it may remain level for a brief moment before gravity takes over and the paper starts to tilt. As it tilts, the weight distribution of the paper will shift, causing it to become increasingly unstable. If the tilt angle is steep enough, the paper will eventually fall to the ground.

In some cases, it may be possible to quickly grab the paper with your other hand to prevent it from falling. However, this depends on how quickly you react and how close your other hand is to the paper.

Overall, the outcome of letting go of one hand while holding a piece of paper horizontally will depend on several factors, such as the angle of the paper, the force exerted by the remaining hand, and your reaction time.
assistant
How long will it take for the paper fall onto the floor assuming the paper is a piece of A4?
user
If the piece of paper was dropped in a vacuum, it would take sqrt(2h / g) seconds to reach the floor. The variables being h, the height at which the paper was dropped from, which depends on your height, and g, the acceleration due to gravity at the location where you dropped the paper.

In practice however, the piece of paper would almost certainly take a longer time than that to reach the floor. An A4 size piece of paper has a vastly different coefficient of drag depending on which way it is facing the airflow as it falls, not to mention that it would be free to bend as a response to the airflow as it would be falling. This makes the actual time to fall down practically impossible to predict. It is very much dependent on initial conditions and the specific air flows in your location.

If you were to perform this experiment and repeatedly measure the times it took for the piece of paper to hit the floor, you would come up with a variety of different values. The vacuum calculation presented in the beginning would give you a solid lower limit to those values, assuming no downdrafts.
assistant