Just hot enough to melt a clean slot at your feed speed — typically a faint glow in a dim room, not bright orange. Too cold and the wire drags and tears; too hot and you get a wide, melted, droopy kerf and lots of fumes.
Control it by varying current: an adjustable bench supply, a PWM MOSFET module (duty-cycle = heat), or a series resistor for coarse setting. The right value depends on wire length, gauge and material, so calibrate: set a feedrate, cut a test slot, and trim heat until the wire glides through leaving a crisp edge. Re-tune if you change wire length, gauge, or cutting speed significantly.
Control it by varying current: an adjustable bench supply, a PWM MOSFET module (duty-cycle = heat), or a series resistor for coarse setting. The right value depends on wire length, gauge and material, so calibrate: set a feedrate, cut a test slot, and trim heat until the wire glides through leaving a crisp edge. Re-tune if you change wire length, gauge, or cutting speed significantly.