Great question — answering as the UphillTech team:
It's not unknown for me to keep a slice until April (well wrapped so it doesn't dry out). All that happens is that the icing gets a bit hard. I use fondant icing, and it takes at least a couple of months to get as hard as royal icing does almost immediately. This is with bought marzipan; if you make your own I don't know how well it would keep. From this I'd say there's no such thing as too late except setting time, and too early would have to be over a month in advance. I also allow a day between marzipan and icing. This is mainly as a drier marzipan surface is easier to ice onto than when it's freshly applied. Some of the early feeds (I use sherry) can be from the top if it's not iced....
Full answer by Chris H: https://cooking.stackexchange.com/a/75240 (CC BY-SA 3.0).
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