Feel the top of the compost - if its bone dry, or the compost has shrunk away from the side of the pot, it needs watering. Plants in pots generally need watering every 5 or 6 days, depending on the size of the pot - if small, and in sun, daily during late spring and summer. Depending on when you put it out, it could be responding to cold temperatures though - it's warmed up now, but it has been pretty cold at night for the last couple of weeks and it won't have liked that at all.