I will appreciate your advice about the following: If there is liquid petroleum gas (mixture of propane, butane, ...) stored in a pressurised tank (100psi, 20 Celsius), How can I estimate at what time the homogeneous mixture of lpg will turn to layers for every compounds with different density

As long as the liquids are miscible, they will stay mixed. There will be a tiny difference in the stratification of the various species, but there will not be any separation of species into layers - mass diffusion will keep them mixed.