Depending on the type of venom, it can be extremely fast. There is a shocking story that Werner Herzog tells about an incident that happened while filming Fitzcarraldo deep in the Peruvian jungle. They had hired a bunch of local natives to work on the production. There was a crew of them downing and clearing trees. One guy was using a chain saw. Out of nowhere, he was bit on the foot by a fer-de-lance. They are known for being very aggressive. The guy with barely a pause started the chain saw and quickly removed the infected leg below the knee. He collapsed to the ground as some as the leg dropped off. The other natives quickly put on a tourniquet and hauled him away. He ended up surviving. He knew in that moment that he had less than a minute or two to save his own life. He did what was necessary.