He had roughly 30-60 seconds depending on the depth and location of barb penetration, and it is likely Steve felt tremendous pain. First from the piercing of the barbed probe. Then from the excruciating pain of live tissue being necrotized by the venom. Then, if he was still conscious, by the pain of a massive, fatal cardiac arrest. Thankfully, as you can read in books such as "How We Die", God has blessed us with endorphins to block out pain entirely during a fatal trauma. Hopefully Steve's endorphins kept his suffering to a minimum.
Once the heart goes into the v-tach/v-fib combo, things are pretty painless because the person becomes unconscious within seconds. I know it sounds perverse, but let's hope that the venom significantly irritated the heart early on in the process so his ventricles fibrillated seconds after puncture and he suffered as little as possible.