How fast can a car go? How much is enough? What is the airspeed velocity of an unladen swallow?
It all depends on your system resources, your internet service, your geographical location to the node, or pool you are using, Your voltage and clock settings, How often, and for how long you are planning on running your mining devices, and the difficulty level of bitcoin. A lot of factors to consider. Stock hashrate is what 15gh/h per device I think so around 75gh? you can google bitcoin mining calculator and enter in 75gh to get a rough idea I guess. Hope that helps!