How long to quarantine/isolate in Spain if test positive for return to the US?

It is up to the local authorities for the most part, based on the symptoms you have and how long they present. Secondarily, since you may still test positive after the quarantine period, you may need a letter of recovery from a Doctor, and the airline will expect some standard period of isolation as well.

In the event of a positive test, either self-test or at a clinic, contact a Doctor immediately (your hotel will help) and get under care. They can document your symptoms, and determine length of isolation, and likely issue a letter of recovery if you are still testing positive. The worst thing to do would be to do a positive self test, tell no one, then go looking for help after whatever period of isolation you may have heard is OK.

According to local guidelines, you're good if mild/asymptomatic: https://www.thelocal.es/20220323/covid-19-spain-scraps-self-isolation-for-asymptomatic-and-mild-cases/

As to where you get a letter of recovery for the flight home and how long to delay your flight if you are so unlucky as to catch covid on vacation, I just wish us both luck. I'm going in May too, and I have not gotten very far in my research. I'm kinda just hoping they'll eliminate the testing requirement, wishful magical thinking got me this far in life...

I've also read accounts, but I have not paid much attention to what they did to get home.

I found this article.
https://www.thelocal.es/20220323/covid-19-spain-scraps-self-isolation-for-asymptomatic-and-mild-cases/

It looks like Spain does not have isolation requirements at this time, although we know this could change. Your "worst" case would be waiting until you either tested negative or could get a certificate of recovery, whichever came first.

There are several place to get a certificate of recovery.
You can telemedicine with your doctor.
You can work with a local doctor.
You can use an online physician service such as this.
https://quick.md/documentation-of-recovery/

va, check your airline's website also. United is saying to isolate for ten days after a positive test (so you can fly on day 11).
https://www.united.com/ual/en/us/fly/travel/notices.html

They give no info on the letter of recovery requirement.

The United States Government asks only for either a negative test or a certificate of recovery.

A big thanks to all who have responded.

I have bookmarked the quick.md site, and will call our medical center tomorrow to find out if they have a procedure.

the local.es seems to be behind a paywall, but I could see a bit of the articles.

We are flying United, so I would have to follow their rules.

So if Spain has eliminated isolation, but we have to wait 10 days for the certificate of recovery, it’s possible we wouldn’t be confined to the hotel room, and could maybe go wander in a park?

We both just had our 2nd booster and mask regularly, so hopefully this is just unnecessary precautions, but it’s more calming to know what might be, rather than be surprised over there.

That is how I am reading it--here is the subtitle from the article, which sums it up nicely:

From Monday March 28th, people in Spain who test positive for Covid-19
but are asymptomatic or have mild symptoms will no longer be required
to quarantine for seven days.