By the time my batch gets it's first check, it's been around 8-12 hours after oxygenating and pitching the starter slurry in... I typically have active fermentation sign by then.
In my latest batch, I'm using a sanke fermenter kit, with thermowell, with a digital probe thermometer down the tube. It has the current, high and low temperatures displayed. It had already peaked by day 4, and was starting to cool down. Really like having the thermowell in there, and not having to rely on the stick on fermometer to be accurate (that's off by a few degrees it seems)...