Depends on how you view it, canonically. The first hinting of it is episode 19 (Ichigo becomes a Hollow!). The first time we see Hollow Ichigo's inner-self form is I believe episode 38 (Desperation! The brokened Zangetsu). The first time the mask actively materializes is either 57 (One thousand cherrry blossoms, crushed! Zangetsu thrusts through the sky) or 58 (Unseal! The Black Blade, the Miraculous Power).