In my recent discussion with my colleague, he said that the 20 ns are good enough to do MD simulation, in another day, someone told me that either 30ns-50ns are needed to determine the complex stability more precisely.
There is no specified time duration for the MD simulation. If you will go through the articles so you will see researchers have done 100ns, 200ns, and even 500ns MD simulation but Journals mostly consider at least 100ns in articles.
The optimum time for molecular dynamics (MD) simulations in drug design depends on the complexity of the system being studied and the specific research question being addressed.
However, if authors have performed MD simulations for less than 100 nanoseconds, it is often suggested to extend their simulation time to ensure a more comprehensive exploration of the system's dynamics. Additionally, if the MD results exhibit significant fluctuations and instabilities, it is advisable to perform MD simulations for more than 100 nanoseconds to achieve a higher level of confidence in the observed behavior.