I have read in the past of using some of the second run from mashing to make a starter. If you mash a 1.046 wort for the purpose of using it in the future for yeast starters (no hops, etc.), how long would you need to boil it? As long as it was sealed, would it keep indefinitely in the refrigerator?
No. You need either to freeze it, or boil under pressure (15 psig) and can it. Storing it in a sealed container at room temperature or in the fridge would be downright dangerous.
I have been (I think) successfully brewing a large batch of starter wort, about 3 liters for the purposed of a two step starter. Actually, I usually brew two batches each time I brew, so I make two starters. After I boil and cool, I make initial 700ml starters and seal the rest in Ball jars. After sealing I spray the tops thoroughly with Star San and refrigerate. Then after the first step of the starter takes off, I decant and split up the remainder.
It depends on how long you store the wort. As long as your sanitation is good, a week or so should be fine. Any longer than that and I would either freeze or reboil it. Especially if it’s in a sealed glass container.