Stand-by time (of mobile phone) is the officially quoted longest time that a single battery charge will last when the phone is constantly connected to the GSM network but is not in active use.
The stand-by time is highly dependent on the cellular network environment, such as the distance to the closest GSM cell tower (base station). Moving the phone (as in a vehicle) also negatively affects battery life.
Manufacturers measure talk time in controlled conditions and the quoted numbers are rarely reached in real-life scenarios. These numbers are best used as reference when comparing phones from the same manufacturer because battery life measurement methodology will probably vary from manufacturer to manufacturer.