Water weight...how much?

I have been away for a week and I just weighed myself and the scales say I have put on 1.5kgs in a week! I don't really understand how I could have managed that considering I ate pretty well and exercised (not as much as usual though). However I did drink more alcohol than usual. I have read on other discussions that water retention can be blamed for some of the weight. If this is true, how much (how many kgs/lbs) can really be blamed on water retention?
Thanks for your help 

I don't for sure but on Friday I weighed 168. Yesterday morning I weighed 175 (omg! 7lbs up!) and today I was back to 168 again.

I'm not sure what the deal was yesterday--7lbs seemed like a LOT of water weight gain to me in just one day!