If we assume most EVs have a 60kwH battery, then the house wall charger will pump out 6-7kW/hr and the 3 pin plug will do 2-3kW/hr. I need (very approx) 24kWh for my shift so you can do the maths from that.
The downside of the 3 pin method (granny charger) is that you're pumping ~12 Amps through the plug for hours at a time. If your plug (&/or extension cable) is not up to it, it could melt the socket, the plug, the cable and I guess set the house on fire! Having said that, plenty of people are happy to do that. I only do it while I'm awake otherwise I think I'd probably be able to 3pin home charge all the time (3kWh x 8 hrs = 24kWh). Realistically though, I don't think your average house wiring/sockets are designed for that sort of continuous ampage. Hence, my split between home charging and external charging.