In Volo's Guide to Monsters, on page 81, the following section is included under the Mind Flayer sub-chapter:
Mind Flayer Magic Items
Some mind flayer colonies have developed the ability to create or modify certain kinds of gear, imbuing them with psionic energy. Mind flayers craft magic items that only they or their thralls can use–a sensible security measure to keep enemies from turning the illithids' own creations against them.
It then goes on to list a few such magic items on page 82, such as the mind blade, but it doesn't seem to go into any further detail on how such items are created/modified, or how long it would take.
My question is: How long does it take for a mind flayer to create such a magic item? If you handed an agreeable mind flayer a non-magical sword, how quickly would the mind flayer be able to hand you back a mind blade?
I'm primarily interested in 5e information, but if there's any information about this from previous editions of D&D, I'll happily accept that as well. Any additional information about how such items are created/modified is welcome, but not necessary (since I'm mainly interested in how long it will take).
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".