A method of determining gelation time is disclosed that includes placing a composite core plug into a core holder of a commercially-available vessel. The composite core plug includes a first core plug with second and third core plugs disposed on opposite sides of the first core plug. The second and third core plugs are saturated with polymer solution, and the first core plug is saturated with gel solution comprising polymer and crosslinker. The method further includes alternating polymer solution injections between a first injection area located on the second core plug and a second injection area located on the third core plug, while ensuring that the polymer solution is being continuously fed to the composite core plug. The pressure drop across the composite core plug is monitored during the alternating injection of polymer solution to determine the gelation time of the gel solution in the first core plug.