Half a litre over that time period.... Just a couple of questions but I will explain why...
1. When you check is the expansion tank totally empty or around the minimum mark?
2. Is there any dry stained runs of coolant around the top of the expansion tank?
What is unusual about Alfa 147/156/GT's from other marques is that that the expansion tank itself is actually not in the best location for the coolant equilibrium of the engine block ranges that they use... The engine is lower than the tank so the block can be full of coolant as is the radiator yet the expansion tank can look very low.
So when you top it up to maximum, natural expansion of the heated fluid occurs and the expansion tank cap does it's job and releases small amounts of coolant until the equilibrium has settled at its nominal level.
Some new owners tend to panic if they aren't used to this quirk in difference of level and tend to continuously refill the coolant to max again, thinking they have a leak.
It took me a while to figure it out too
You have to understand that Alfa's use a lot of Fiat parts bin equipment in order to keep costs down for their parent company. So sometimes the parts they use aren't the best shape for their location. Saab cars that used Vauxhall parts had the same issue btw.
Fill it to half way between min and max when the engine is warm, being careful when opening the filler cap. The level should jump up all of a sudden. Check it again after a run out a day or two later and the same should happen. If so, you have plenty coolant in and not actually losing any.
Hope this helps and don't automatically assume it's got a leak, few owners have actually figured this out yet and still search for leaks in vain.