Calculate the frequency, period and duty cycle of a 555 timer in astable mode, or the pulse width in monostable mode, from the resistor and capacitor values.
Astable: frequency = 1.44 / ((R1 + 2·R2)·C), and the duty cycle is always above 50% because the capacitor charges through R1+R2 but discharges through R2 only. Monostable: pulse width = 1.1·R1·C. Standard formulas for the classic 555 IC.
Astable makes the 555 oscillate continuously, producing a square wave (used for blinkers, tones and clocks). Monostable produces a single timed pulse when triggered (used for one-shot delays and debouncing).
Because the timing capacitor charges through R1 and R2 in series but discharges through R2 alone, the high time is always longer than the low time, so the duty cycle is above 50%. A diode across R2 or a different topology is needed for 50% or below.
Keep resistors between about 1k and 1M and the capacitor above roughly 1nF for stable operation. Very small or very large values make the timing sensitive to leakage and tolerance.