The DMs guide has little information on magic item crafting. Xanathar's has more guidelines, recommending (among other things) that the time and cost is based on the item rarity.

E.g. Rare = 10 weeks and 2000gp, Very rare = 25 weeks and 20,000gp.

One of the "other things" the guide also suggests is questing to gather rare material components etc.

These guidelines are really only for player characters though. For an NPC - especially one crafting an item special to their race such as the mind blade - its going to be entirely up to the DM. Even determining the rarity is going to be a judgement call (a mind blade is going to be extremely rare in general, but perhaps merely "uncommon" within mindflayer society).

Conclusion: it takes the "speed of plot".

The DMs guide has little information on magic item crafting. Xanathar's has more guidelines, recommending (among other things) that the time and cost is based on the item rarity.

E.g. Rare = 10 weeks and 2000gp, Very rare = 25 weeks and 20,000gp.

One of the "other things" the guide also suggests is questing to gather rare material components etc.

These guidelines are really only for player characters though. For an NPC - especially one crafting an item special to their race such as the mind blade - its going to be entirely up to the DM. Even determining the rarity is going to be a judgement call (a mind blade is going to be extremely rare in general, but perhaps merely "uncommon" within mind flayer society).

Conclusion: it takes the "speed of plot".