How long does eggwash keep? I don't use eggwash often- roughly once a week I make a speedy en-croute or pie with some shop-bought puff. Whatever I make, as it tends to be just for two, I rarely use more than half an egg's worth, and it feels a bit wasteful throwing so much away (for this reason I tend to use milk). 

If I use a fresh egg (with at least a week to its expiry date), can I keep a cup of eggwash under cling-film for a week? If the expiry is further away, will that make a difference?
To make a long answer short, if you are using it once a week, then don't bother storing the egg mixture. The egg wash is definitely not safe after a week even if you store it in an air type container in the refrigerator.

If however you are using just egg whites, you can lightly beat half of it and use that and freeze the other unbeaten half in the freezer. 

But to be honest... There are worse things to waste than half an egg a week. I think this is a case of not being worth the effort.