Your device screen light is turned off after a certain time your device is not in use (the time varies between device models), this is designed specifically for your device to save the battery power. It is not possible to change it. But you can turn on the light easily by pressing any key.
Note: the screen saver can be turned on or off separately in Menu > Settings > Display > Screen saver, select a screen saver and the time-out. The screen saver is activated in standby mode after the defined time-out. After the time-out of the screen saver, the power saver will be automatically activated.
Nokia 2700 and 2730 phones do not have screen saver.