The time constant τ (tau) of an RC circuit is defined as:
τ = R × C
Within the time τ, the capacitor charges to approximately 1-1/e = 63,2% of the supply voltage.
After 5 × τ the capacitor is charged to around 99%.
In theory, a capacitor never reaches 100% charge, but in practice, it's considered fully charged after 5 × τ.
Example:
R = 47kΩ
C = 10µF
τ = R × C = 47000Ω × 0,000010F = 0,47s
Therefore, the capacitor is charged after 5 × τ = 5 × 0,47s = 2,35s