We determined that his particular set or armour was Very Rare, being +2 and having a cool power, which, according to the DMG meant he would take literally years to complete his work! Obviously this was stupid.
So, here is a slight tweaking of the official rules, to bring a little variance to the process, and to bring those times down to something reasonable.
Unless it's mentioned here, the rules in the DMG are unchanged.
To determine the time taken to make a magic item...
1) Make an ability check for your spell casting attribute. Add Arcana or Religion if felt to be appropriate.
2) Divide the cost of the item you are making by the result. (see the DMG, page 129)
3) Divide this by 25. This is the number of days you need to make the item. Round up. Making an item takes at least 1 day.
4) Artificers take half as long to make magical items.
By the way, we are getting to a point where I should be ready to share the full 20 level Artificer class with you all. We have play tested two of the three builds I came up with (Warsmith and Tinkerer), and so far, apart from a few adjustments, they are proving pretty balanced and playable.