In order for a nether fortress farm to be most efficient, you have to prevent mobs from spawning within a few chunks of the fortress.
What is the easiest way to do this in survival, and how long would that usually take?
"within a few chunks" is actually within 128 blocks (euclidian) of any point of the farm if one wants absolute maximum efficiency: the same from the player position for just slightly less. [Whether accepting most of the maximally attainable rate in exchange for considerably less work is a personal choice…]
You'll probably end up using a mix of methods…
Firstly, careful selection of which nether fortress (or part thereof) one uses can save a great deal of effort.
Conventional wisdom is to look for a fort that generated in clear air above a lava ocean; the lava is non-spawnable and generation in the clear minimizes the number of blocks needing removal. [The contraian move is to use a fort that generated inside the netherrack; this can make construction easier/safer and is spawn-proofed finding and filling (or slabbing/buttoning) any nearby caves]
Almost certainly a large number of blocks will need to removed (of placed if one spawn-proofs by filling-in). A mix is also possible, leaving any large solid blocks (filling any small caves within) and removing smaller 'islands' [This may be more efficient than relying solely on either technique.]
Lava can be placed and allowed to flow over areas rendering them spawn-proof.
Finally, there will be some residue of blocks that need to be spawnproof, but accessible to the player (walkways and so forth). Stone slabs or buttons are two obvious alternatives; neither is flamable, either can be walked on and both are cheap nonespawnable blocks.
Slabs are actually cheaper than buttons (2 for 1 block of stone vs. 1 for 1 with buttons) and can be made of cobble (which may be marginally cheaper yet), but require care in placement to avoid leaving unclimable 1.5 block steps..
How long such a project would take [probably best considered in terms of hours of play] is dependent on a number of factors:
How does the individual play? (Some people have a more focused playstyle and will consequently work more swiftly.)
What is the location like? (The more air and lava ocean one starts with, the less needs to be done.)
What resources/'technology' does one have available? (Having Efficiency5/Mending diamond tools, an Ender Ender (or other high efficiency means of repair), plentiful ender pearls, high end armor [to better fight off mobs], an efficient stone generator, elytra [mainly for safety], varoius potions [Fire resistance and night vision most obviously] , and shulkers [for transport] all speed matters considerably.)
How much help is available? (If there are other building parts of the supporting infrastructure eg. the Ender Ender, or giving/selling)
RE the above post:
A quick search turned up HermitCraft 6: 67 | 50000 BUTTONS! which shows such a farm (built in a fortress thart looks to have generated in air above a lava ocean) and uses ~45,000 buttons. Built by iskall85, he notes that he is proud to have built it in "less than 5 days".
Assuming 10 hours a day (not unreasonable for a professional player) this would be 50 hours.
One can safely assume skill, experience, and top level gear… He also had considerable help; both in direct donations (eg chests of stone) and others building/maintaining various supporting infrastructure.
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.