Smoothed histograms of the number of years required to test for Earth-like O₂ levels on hypothetical Earth-sized habitable-zone planets using planned ELT telescopes and theoretical very-large-aperture ground-based telescopes, assuming full transits (upper two panels) and partial transits (lower two panels) are required for measurements. These calculations are based on a Monte Carlo simulation of M-dwarf systems within 20 pc, randomly sampling a fraction of stars and their hypothetical planets 10,000 times, accounting for both geometric transit probability and η _⊕ from Dressing & Charbonneau (2015). The median values of the corresponding color histogram are shown as circles on the x-axis. Note the different scales along the x-axes.