How long will a yeast last

did made a extra yeast starter four days ago a just in case starter i did put it into the fridge but how long will a yeast starter last before it goes bad in the fridge
tomorow make a other starter for my friday brew day

Your yeast will not go bad if it is in a sealed container. The normal kitchen refrigerator does get a bacteria build up over the years of use. Sanitized foil held tightly in place with a rubber band is good for one or two weeks. Screw lids or two piece canning jar lids are best for longer storage.

The bacteria that may get into yeast stored for the short term is not a killer for your next beer. The small amount of contamination will not survive in a fermenting wort. Infections in yeast that is stored for months will become visually evident. Normally though infectious material that will sour your beer, such as lactobacillus are not in our refrigerators unless we also store unsealed grains or sealed bags with the exterior contaminated.

Stored yeast will lose viability though. Starter yeast stored under water could lose up to approximately 14% viability in a couple weeks. Stored under wort perhaps only 5% to 8%. These are just approximate numbers. It is just that yeast stored under the spent wort will lose less viability over time.

I always build my starters so I will be pitching more yeast than a pitch rate calculator will suggest. This way I don’t have to worry about a starter not having sufficient viable cells if I pitch a couple weeks later.

that was my idea as well pitch more yeast
i do use a mason jar for the extra yeast