Citrus - When to water?

I got my citrus in August and it started to flourish. I just got back from a business trip and found that the leaves are wilting and the one small lemon I had has dropped off. my instinct is to give it a good water but am a little paranoid about over watering. It is kept in the coservatory and gets good light So I don't think its the light. I last watered it around 3-4 weeks ago.

Any advice gratefully received!