So spend a month for 3,000 meters and two months for 5,000 meters? That's matchs what I've read and experienced for FULL acclimatization meaning your red blood cell count has fully adjusted to your new environment in addition all the quicker adaptations. Few of us, though, get to take that much vacation time.
And as Bob Gross mentioned, above 18,000 feet, all bets are off. Below 18,000, more time means gives you more acclimatization. Above 18,000 and you just get worse, your body doesn't heal and "climb high, sleep low" is the only way to go.
But for our more typical trips - going to 3 km or 4 km from sea level, your blood pH, beathing tidal volume, and an initial thickening of ones blood happens over the first few days. While it takes 6 weeks to be 100% acclimatized, I'd rate myself at 20% for having spent the night at the trailhead, and 40% at 3 days.
Two easy to remember data points on atmospheric pressure:
At 7,500', there's 75% of sea level air density.
At 18,000', half the atmosphere is below you (50% sea level pressure).
"Put your own oxygen mask on first. Then assist your companion."