How many days in the future do extended forecasts become useless?

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They have those 15 day extended forecasts but can you really take any prediction past a week or so out with any confidence?

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Anything past a day or two is unreliable

Anything past 15 min

Within reason, it's useful. Look at the pattern, not the forecast for a specific day.

like 3-4..

After 2-3 days, they give a general idea of what may happen in the 48-72 hr period surrounding a day, but little in planning a morning or afternoon event.

Depends on where. Some places are easier to forecast and models agree more frequently and further ahead than they do for other places.

In general though, beyond 2-3 days is a good general rule to consider a forecast not very reliable. For up here in the mountains, beyond 2 days isn't worth planning around. For Houston, 3 days out is usually somewhat reliable in a general sense. No matter where though, if timing and amount of precipitation is what you need, the day before is best of course.

ETA: NWS re-runs models and updates their forecasts multiple times a day, I think every 6 or 8 hrs if I remember right.

1-2

Opensnow.com is a good supplement to NWS for CO snow forecasts a few days out. I use it to see if a system might be coming through 4-5 days out and check it once a day until it's 24 hrs before when I want to know specifics. Then I'll use NWS within 24 hrs for specifics.

Temperature and more importantly snowfall.