From the Guinness website:

From the tap – “…GUINNESS® Draught is best served at 6°C (that’s 42.8°F)…”
Guinness Draught in a bottle – “…Just chill your GUINNESS® Draught in the fridge for at least three hours…”
Guinness Draught in a can – “…Chill your GUINNESS® Draught can down to about 3.5°C (38.3°F) for at least 3 hours…”