How many days should I keep the idol?

Commonly 1.5, 3, 5, 7, or 10 days; choose as per tradition.

What to do with nirmalya (offerings)?

Compost/eco‑dispose respectfully; avoid water pollution.

Install Lord Ganesha idol on a clean altar, offer durva grass, modaks, and flowers, chant Ganesh mantras, and perform aarti daily until visarjan.