When will my beer taste consistent?

Welcome to the BA site, Steve. Your question is a good one, and is difficult to answer because an answer likely is going to be different with different styles. I think all styles have a 'green' period where they still need to mature, so there's that. An answer to your question as it applies to a single batch of homebrewed beer should be easier, but outside factors (food just consumed, something still happening inside various bottles other than aging, infection) could be at work here that can have an affect and throw off your taste experience.

The only style that I can comment about from experience is the IPA. In my opinion, that style does not reach a mature taste until 3-4 weeks after packaging, so young versions of IPAs will taste better after a few weeks and stay that way until the hops begin degrading. If your beer is a hoppy version, maybe that's what is happening.

 

If the carbonation often varies, I would think about adequate mixing of the priming sugar when bottling. Also, a high amount of sucrose may lead to a cidery flavor. Just a thought.

 

Points made by @PapaGoose03 about " ... food just consumed ... something still happening inside various bottles other than aging ... infection [sic] " are spot on.

My raspberry brews reach their 'Peak Flava-Flav' between seven and nine weeks in the bottle. They're certainly drinkable a little before and somewhat after that window.

Also suspect a period of cold conditioning longer than few hours would be helpful.

 

Never brewed a kolsch, but in general, as a beer ages it will taste different from the bottleing buck to the last pint poured.

Hops, malts, wood or other additions all age differently over time.

A fresh IPA say 3 weeks old will not taste exactly the same as the same beer 3 months later.

Even more so when wood aging a imperial.

I think when a beer becomes perfict for your taste, consume it and know over time it will change a lot or just a bit depending on the brew and time.

 

It's not clear to me how much time passed between your first sampling and the disappointing one. It's also not clear what temps your beers were stored at. But I think time and temperature could be an issue.

Flavors fade with age due to oxidation. The rate of oxidation increases with temperature. If you can store all your beer cold, it will be more stable, all other things being equal.

You also mentioned higher carbonation in the older sample. That sounds like increased biological activity to me. This could be that when you first sampled, all the priming sugar had not been consumed by the yeast, and the second time it had been, so more carbonation. It could also mean that the beer you bottled was not fully attenuated, so the yeast had more time to act on residual fermentables.

Another idea related to increased biological activity is bacteria or wild yeast contamination. This includes organisms that can break down and metabolize long chain sugars that yeast cannot. Several of my early batches of beer suffered from infections taking over in the bottle. This always resulted in higher carbonation, which over time would develop into gushers. It also meant that the flavor of the beer was subject to change. My first oatmeal stout gradually developed into gushers, and in the process, the flavors became increasingly duller.

Not sure if this is at all what you are experiencing. You said mustard, and I've never heard a beer off-flavor described as mustard. Of course, mustard is typically acidic, and acetic acid can result from contamination.

Although I can't diagnose your problem, I advise to pay attention to bottle cleanliness and sanitation, and store your bottled beer warm only until priming has peaked, and then move into cold storage.

 

Not sure which question or problem to answer towards because it sounds like a few might exist concurrently.
Could your carbonation be inconsistent because your primer wasn't mixed into solution? Sounds like it could be.
Could it have picked up additional fermentation because whatever you pulled through had something worth paying attention to? Maybe?
Could the bottle have a funny taste because it wasn't cleaned thoroughly and some residue from whatever it played host to was stuck inside? Maybe? I've had that problem a couple times, but I always get a pleasant peach-ish like off flavor.
Flavor consistency kind of depends on what you are brewing. But, 2 - 3 weeks is about normal for most everything.
Just be sure when you are adding things post fermentation and into the bottling process that you put it all in BEFORE you rack over it, and that you have enough hose to coil and to enable circulation of your liquids so that everything can be put into solution consistently.

 

Mustard flavors and increased carbonation? If the timeline was longer I would say infection. But infection doesn't move that fast.