How long it'll take is based off of how expensive it is, but you effectively make it for 1/10 the price (see craft skill). The breastplate is 4200 gp(mithral + breastplate) so crafting it's 4200 sp. Every week you take a check and if you succeed, you complete the result X DC in sp.
Rolling well, this is going to take you about 3 months to make. Yeah. Suggest throwing in item creation buffs spells to speed up the process. You can also voluntarily increase the DC to build faster.
I can craft an item for 1/10 the normal price? I can't find this in the Core Rulebook.
I found this:
1. Find the item's price in silver pieces (1 gp = 10 sp).
2. Find the item's DC from Table: Craft Skills.
3. Pay 1/3 of the item's price for the raw material cost.
So, would it be 42000 sp? Which means a massive amount of time! ( Who would create a Luck Blade???)
And to buy the mithril would cost 1400 gp (1/3 for the raw materials)?