How long to get new plate mail?

Does the blacksmith have a full set of armour he can just sell you off the shelf?

In mediaeval reality the answer to that would be no, as armour worth its salt has to be tailored to the individual. Yet in every fantasy RPG I've noted, they just give you prices with no delivery time. 

Now consider it takes a long time to make armour, even using modern machine shop tools which can be months. Consider doing that with a blacksmith's hammer and anvil, it'd take an age. Mind you advanced mediaeval blacksmiths would have access to water-wheel driven power hammers which would help. So lets assume that the PC can return to the armourer for repeated fittings, 

I estimate that it should take about 6 months to produce a full suit, provided that the armourer is ONLY working on that one suit and is provided with access to all the quality metal he needs. This scenario is only likely to be the case if the armourer is in the employee of the PC, your average trader will be having to service other customers in order to make enough money to live, and this could easily double the time it takes to make the armour.

Of course someone at sometime would have been knocking out mail shirts with a one size fits all, yet I think this should come with a penalty, perhaps a -1 DEX mod. If the armour isn't tailored then that crease, or fold, or articulation is going to be in just the wrong place!

Okay, lets assume that the local armourer has 1 or 2 helmets for sale, perhaps a set greaves, maybe one gauntlet from a customer who died before paying for the second one, so where does that leave you when trying to work out the AC? Clearly you shouldn't get the benefits of full plate, but a few solid sheets of metal over some vitals should offer some in-game benefits, and do so in a way that doesn't slow the game too much.

Lets assume you PC is wearing a full suit of chain, but has started adding elements of plate armour to their gear. A quick way to do handle this could be to roll a D6 at the start of every battle, scoring a success on the die means the PC uses a slightly higher AC for the duration. What constitutes success on that die, could change over time depending on what upgrades there have been. 

Greaves 1 in 6

Greaves, vambraces  2 in 6

Greaves, vambraces, helmet, 3 in 6

Greaves, vambraces, helmet, chest plate 4 in 6

...and so on.