Time the output stays HIGH after a trigger pulse.
Astable Mode:
$T_{high} = 0.693 \times (R_1 + R_2) \times C$
$T_{low} = 0.693 \times R_2 \times C$
$Frequency = \frac{1.44}{(R_1 + 2 \times R_2) \times C}$
Monostable Mode:
$T_{pulse} = 1.1 \times R_1 \times C$
Ideal 1/3-to-2/3 supply threshold timing: astable tH = ln(2)(R1+R2)C, tL = ln(2)R2C; monostable t = ln(3)R1C. Actual timing depends on IC thresholds, discharge voltage, leakage and component tolerance. Check R/C ranges in the TI LM555 datasheet.