CMOS stands for “Complementary Metal Oxide Semiconductor”.
It is also known as Non-Volatile Ram (NVRAM) or Real-Time Clock (RTC). A Complementary Metal Oxide Semiconductor is a battery powered chip that provides start up information like Hardware settings, system date and time settings etc to BIOS (Basic Input Output System). BIOS uses this information when you start your computer.
Generally, CMOS battery can last for 10 years. But that depends on certain factors, e.g environmental factors, overheating can damage or reduce its lifetime.
A CMOS Chip has approximately 64 Bytes of memory.