How long does the Truffle take to move in after building the mushroom biome?

It isn't easy to tell if the biome you are in is surface or not because of the walls behind the house, I suggest breaking all of that. Also, as it states in the wiki, "to get the Truffle to move in, there must be housing in a surface Mushroom Biome during hardmode." judging by the list you have provided you have done all of the things that are required. All that's left is to see if the house you built is even suitable for NPC's.

As it states under the NPC section of the wiki, a suitable housing has:

• Proper Lighting
• Enclosed frame composed of blocks, a door, and walls. (Size: About 5x12 blocks)
• 1 Flat Surface Item (Ex: Table, Dresser, Bathtub, Bookcase, Workbench, Piano)
• 1 Comfort Item (Ex: Chair, Throne, Bench, Sofa, Bed)

From the pictures you gave, the program might be mistaking your Surface biome as an underground one because of the excess grass walls or not enough lighting. Another thing might be the height of the room.

If nothing works, just have a little patience, NPC's sometimes take a while to move in. Also, just try to preoccupy yourself with something else while waiting to help pass the time. Another thing, those walls inside the house must be player placed, if they spawned that way (Which I doubt, but doesn't hurt to inform you) you need to remove them.

Please Note: that there is a bug where if you killed the boss Plantera before the Truffle moving in, he will not move in and you will need to make a new world