It has to be left long enough for the hardening reaction to get started so it properly bonds to whatever its applied to.
But you don't want to leave it so long that there is a semi hardened layer between the two parts.
20 minutes is long enough to get the reaction going but if the surfaces have become tacky before your 20 minutes are up then the parts should be assembled immediately to ensure you get a homogenous gasket layer.
Actual time to tacky will be fairly variable depending on thickness, temperature, humidity, substate et al.
Clive