Craig is right about opinions. The common saying is ask 10 winemakers, get 11 opinions!
Reds in the barrel stay there until the next year's wine is ready for the barrel, so approximately 12 months. I have neutral barrels so I'm using cubes for oak flavoring -- I currently use a smaller amount than previously, and leave the cubes in 6+ months. For reds in carboys, cubes set for 4 to 6 months, and I bottle after a total of 6 to 12 months, typically around 9 months.
Whites age for a total of 4 to 9 months. They typically have a shorter shelf life and are drinkable sooner, so I bottle sooner. If adding oak, say to a Chardonnay, the aging time will be longer.
K-meta every 3 months. I was going a bit longer, but my understanding of how SO2 protects the wine by binding with contaminants has increased, so I'm not stinting on sulfite.
What to taste for? Oakiness, flabbiness, off flavors.
I sample the barrels when doing the monthly topup, mostly because I can, not because I need to. In addition to the Angels' Share, I "lose" an extra half-glass of wine each month.
Quality control, ya know!
Cubes in a carboy? I taste test every ~3 months. In the past I used 2 oz cubes for 5 gallons, but I've reduced that to 1-1/4 to 1-3/4 oz, and the heaviness of the wine is a driver, e.g., heavier wines need more oak. While I've oaked heavier in the past, my current mantra is that oak is a seasoning, not an ingredient.
One adjustment I make is more acid if the wine is flabby. I go light on the addition, adding only 1/4 the amount I think it needs, stir, and let it rest for 2 to 4 weeks. Taste and repeat if needed. I discovered that, at least with acid, adding/stirring/tasting doesn't work. The acid needs time to meld with the wine.
Note on tasting -- stir the carboy gently 1 hour before tasting, so the wine is mixed. If you have cubes (or whatever) on the bottom, the taste will be stronger near the cubes, so the wine tastes different depending on where you draw the sample from.