After guests leave, we clean and make up the room, including the bed. Neither of us can stand a room with a bare mattress.
How long would you consider an unused made up bed to be clean?
Around my neck of the woods, about a day 
Between the dust and the insects and the gecko poop, I would have to cover the bed with something that I then removed, to keep it clean. So my preferred method of dealing with the bed is to leave the used bedding on until I am ready to clean the room and remake the bed with clean linens.
But in temperate climates, I would think a freshly made bed would be good to go for quite awhile.
If you can’t stand the look of a bare mattress, why not just have some sort of cheap throw you put over it if you’re concerned about the bed not smellling fresh?
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I’d consider the part that’s slept on to be “clean” indefinitely. But dust settles, there might be a dead mosquito or other bug. Here stuff seems to come out of the evaporative cooler vent regularly so in summer, definitely the top layer needs to be “dusted off.” I gather up the top layer and take it out and shake it or toss in the dryer to dust off. Muddy’s idea of covering it with a old sheet seems like a good one.
Note your title says a “used” bed which of course is a different answer. You might want to edit.
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I rotate two minimal bedspreads on mine so I probably would leave the clean sheets indefinitely wash or swap out the dusty bedspread. Everyone has to adjust for unusual weather or common pests that can affect how well my approach works.
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Two days for us 
no more. We’ve got the Jerez dampness to contend with also, sadly.
JF
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Everything but the cover is probably technically clean. I have a fear of, I don’t know, something getting inside between the sheets so we always make the bed the day of the guest arrival. However, it’s probably better to go with reason and not worrry about it. The sheets are clean if they haven’t been slept in.
(Though I just got the one-dead-mosquito song back in my head, help me @aelilya 
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Soy un invitado sincero
Viajo por el mundo
Soy un invitado sincero
Viajo por el mundo
Y solo daré una estrella
Sábanas polvorientas
Crusty, dusty sheets
When did they make this bed?
Crusty, dusty sheets
Or is it just in my head?
I’m sorry, @JJD. It had to be done. 
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Here, in winter, the bed in our listing is freeze dried, so to speak. The space is heated with a woodstove, unused when no one is around.
One of the stay-at-home orders lasted from December to the end of March, I think it was. I had cleaned and made up the bed for a December guest who had to cancel to comply with the order. Next guest, after many such cancellations, was first of April.
So my answer is around four months…
I did inspect the bedding when I cleaned, but it was perfectly fine. Five stars. Sparkling (icy sparkles) clean.
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Like others said, depends on location? Ours is in the woods so dead bugs accumulate FAST.
I’d pick up some painter’s plastic sheeting or cheap tarps and throw them over the beds if they’re made up/clean but not being used for a while (“a while” being totally dependent on your particular location and how fast undesirable things accumulate).
An extra sheet larger than the mattress size or large cheap blanket might work too.
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I wouldn’t use plastic. Could get moldy- funky under there.
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Yes, that could be a problem in a humid area and/or over a very long period of time. It’d be ok in my climate for a week or two, but big blanket or sheet would be safer.
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