How long does vaped weed stay good to eat for?

Weed can keep its potency for years.
Assume the herb is already decarbed, and about half as potent as regular herb.
Here's what I do, which is a shortened version of BrassNwood's excellent methods.
(1) Grind the already-decarbed herb to fine powder.
(2) Mix the desired amount of herb into the oil of any recipe, along with about 1/4 to 1/2 as many grams of lecithin.
(3) Quickly pre-heat this mixture to 220 F, then bake at that temp for 20 min.
(4) Freeze the mixture for a few hours.
(5) Thaw, mix into the recipe, and bake.

Alternatively, the herb can be used as is without doing steps (3) - (5), but the result will be slightly less effective.

Each gram of good 15% thc herb used would supply about 150 mg thc to the finished product, but the vaped herb probably has only about 70 mg thc per gram.
For example, using 17 grams of your abv to make 30 cookies would give each cookie 0.57 gram abv, which contains about (0.57 g)*(70 mg thc/g) = 40 mg thc.
Each cookie would contain about 40 mg thc, and each quarter cookie 10 mg.

I've done this with decarbed trim, which is also about half as strong as flower, and there isn't too much herb taste.
But herb does bother some stomachs.

Less herb taste can be had by making tincture first, but that's a lot of work.