Sourdough starter

Please... Which starter? Condition of starter?  How exactly fed? Temperature?  The more details the better.

or is the starter "winging it" with you?  I ask because reviving a dry starter can often be overfed in the first few days.

Discarding can be equated to "dumping the baby with the bathwater" or diluting the starter.  It is not uncommon for wild starters to look active the first 24 hrs only to discover this is only a first bacterial reaction to water.  Try setting up a control, the same flour and water jar without the starter and compare them.  This control may come in handy to judge the new starter.

I parsed your question differently than Mini.

I suppose you are asking:  How long before the strains of yeast and LAB from the flour that I use to feed my starter will dominate (outnumber)  the original strains of yeast and LAB that were originally present in my purchased dried starter?

Implicit in your question (as I understand it), is that you want to keep the original strains as long as possible.

The answer is, you probably guessed it... "It depends."

Here are the four main things it depends on.  1 and 2 are the most important/influential.

1. What you feed it.  Feeding it a white/refined flour like north american AP or bread flour adds as little "outsider" or "newcomer" strains as possible. Whole wheat flour, or whole rye flour, because of the bran, carries more yeast or LAB (i forget which) than white flour.

2. Feeding ratios.  Feeding it high ratios like 1:4:4 adds a higher proportion of those "newcomer" strains, in relation to the originals. So maintenance-feed no more than 1:1:1.   If you need to feed a higher ratio to make a levain for a bake, or use whole grain in your levain, separate off some starter, just enough starter to make the levain for that bake. Keep back a portion of the original starter alone/separate, which is always fed 1:1:1.  Your "mother" starter should then always be fed 1:1:1 with white flour.

A feeding ratio of 2:1:1 , also written 1 : .5 : .5 , would be even better to make sure "newcomers" stay at a minimum.   I think 2:1:1 is as low as you want to go.

3. Cross-contamination. Hands, utensils, and containers can transfer newcomer strains of yeast and LAB. Especially if you have more than one starter in your kitchen or fridge.

If you mill grain into flour at home, that might create some airborne bran dust which can carry LABs and yeast spores and then get on hands, utensils, and containers, or directly transfer via air.

Volunteers for Carl's 1847 Oregon Trail Starter are required to have no other starter or culture in their house, so as to avoid cross-contamination.

4. Temp.  Different strains can prefer different temps. So if the strains you bought prefer room temp, be sure to leave the just/fed starter at room temp for a few hours before refrigerating.  Admittedly, this is difficult to know... what temp the "originals" preferred, and if it's different than the "newcomers" that arrive with every feed.  But I'm just mentioning this for the sake of completeness.

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To give a number...   (I've used 3 purchased starters. I accidentally killed one and lost its backups, and just plain didn't like the taste/smell of the 2nd)   ... I'd say at least 2 years, which is the longest I've had a particular one going.

Carl's 1847 Oregon Trail starter has been kept pretty much the same, supposedly,  since.... 1847.

HTH

When I started feeding my less-vigorous starter with unmalted AP flour, it got a little sluggish, taking longer to peak, and not lasting as long between feedings.

I went back to flour with added malted barley flour in the ingredient list, and that seemed to be a fix.  One can see the added malted barley flour or added amylase in the list of ingredients on the package.

In the US, white (branless) flour needs a little malted barley/wheat flour or amylase added to assist in the breakdown of starch into maltose and glucose for yeast food.

Sometimes I add a little brown sugar or maltodextrin powder to my starters when they seem sluggish.

The benefit of feeding some whole grain flour to your starter is that the bran in whole grain flour brings in extra enzymes to help convert starch fo sugar.

If the white flour in your country does not have some added malted barley/wheat flour or amylase then you may need to add a teensy bit of whole grain flour, or malted barley flour, for the enzymatic effect.

I've been told that EU-grown white flour (usually?)  has the needed enzymes naturally   occurring in it in the endosperm, because they have different strains of wheat than North America.

How much whole grain you can add, and still avoid the "newcomer" yeast/LAB from taking over, I don't know.

I'd love to hear from Debra Wink about this question.  I have wondered this as well what happens to the microbiome of our starters.  Do they stay consistent or do they evolve?  I think with larger ratio feedings especially with whole grain flours there is a greater chance of evolution.  However, the pre-existing microbes have quite the head start and if they are most fit for that composition of the flours, hydration and temperature they will likely persist.  If they aren't most fit for their environment and the flour contains microbes that are better suited then over time I would expect those newer microbes to take hold.

The main recipe I make includes a sponge of 240 grams flour, 300 grams water and a table spoon of sourdough culture. If I make the sponge with AP flour it tastes totally different then when I use whole-wheat for the sponge. Once the final dough is made both contain the same amount of each ingredient but the flavour is distinct. When the sponge is made with whole-wheat the tangy flavour is dominant while the AP sponge bread has a mild subtle sour flavour that many don’t seem to even detect. This seems to indicate to me that different types of flour encourage different yeasts and  bacteria found in my starter to flourish. Same ingredients but totally different results.