I sent of a stick of RAM via Royal Mail first class, they received it the next day and i had the replacement the day after that, so 3 days in my experience from start to finish.
I think it depends exactly what needs to be done/investigated/replaced etc as to how long it will take.
I shouldn't worry about it still saying 'awaiting return of item', all that normally means it that they haven't had a chance to look at it yet and actually start looking at it properly.
I know it might not be of any help now but could be for future reference. I remember someone contacting Coolit directly and getting a replacement within days. http://www.pcspecialist.co.uk/forums/showthread.php?13285-so-my-eco-water-cooler-decided-to-blow-up/page6&highlight=coolit
Considering the rapid response by Coolit, I am pretty sure you should get a replacement in a few days time, from my experience the delay is usually just waiting for someone to put your unit in their work bench.