Lerp shader

I have a shader that lerps between two textures. How would I add deltatime to make it ping pong the _Lerp value?

Shader "Custom/Lerp"
		_Color("Main Color", Color) = (1,1,1,1)
		_MainTex("Base (RGB) Trans (A)", 2D) = "white" {}
		_AltTex("Alt (RGB) Trans (A)", 2D) = "white" {} // Add another texture property
		_Lerp("Lerp Value", Range(0.0, 1.0)) = 0 // Add an interpolation property

		Tags{ "Queue" = "Transparent" "IgnoreProjector" = "True" "RenderType" = "Transparent" }
		LOD 200

		#pragma surface surf Lambert alpha:blend

	sampler2D _MainTex;
	sampler2D _AltTex; // define the properties in the shader program
	fixed4 _Color;
	float _Lerp;

	struct Input {
		float2 uv_MainTex;
		float2 uv_AltTex;

	void surf(Input IN, inout SurfaceOutput o) {
		fixed4 c1 = tex2D(_MainTex, IN.uv_MainTex); // sample a color from the main tex
		fixed4 c2 = tex2D(_AltTex, IN.uv_MainTex); // sample a color from the alt tex
		fixed4 c = lerp(c1, c2, _Lerp); // interpolate
		o.Albedo = c.rgb; // apply the color
		o.Alpha = c.a;

Shaders don’t have a good concept of cross-frame memory and as such delta time wouldn’t help you here - however all Unity shaders automatically get passed the _Time variable which contains the time since startup, and using that in combination with a sine function will allow you to transition back and forth between the textures;

// sine is in range of [-1,1] so remap to a range of [0,1]
half4 c = lerp (c1, c2, sin (_Time.x * _LerpSpeed) * 0.5 + 0.5);