How faint your telescope can see: the classic 7.7 + 5·log10(aperture in cm) estimate, plus the light grasp versus the naked eye.
Classic estimate: limiting magnitude ≈ 7.7 + 5 × log10(aperture in cm). A 150 mm scope reaches about magnitude 13.6. Light grasp = (aperture ÷ 7 mm)². Real limits shift with sky darkness, magnification, transparency and your eyes; under bright suburban skies expect 1-2 magnitudes less.
A common estimate is 7.7 + 5 × log10(aperture in cm): about magnitude 12.7 for 100 mm, 13.6 for 150 mm and 14.7 for 250 mm, under dark skies with averted vision at moderate magnification.
How much more light the aperture collects than your 7 mm dark-adapted pupil: (aperture ÷ 7)². A 150 mm scope gathers about 460 times more light than the naked eye.
Because the true limit depends on sky brightness, magnification, transparency, and the observer. Formulas differ by half a magnitude or so; treat any of them as a ballpark, not a guarantee.