When should I replace my foundation sponge? Please give me a rough idea.

Items made out of polyurethane such as sponges or eyeliner applicator tips will deteriorate gradually from friction, sunlight or how they are washed.
Of course replace your sponge if it is weak and falling apart, but it’s better at the stage when the surface is powdery, or it starts to loose its elasticity.
Carefully wash the dirt from your sponges and applicator tips to make them last. When washing, press lightly into some neutral detergent, then rinse thoroughly and dry in the shade (not direct sunlight).
Use a custom cleansing liquid if available.
If you want to make your make-up look its best, always make sure your bits and pieces are kept clean.