That is correct psubiaco. The uC should have a very low power consumption when the LED is off so the battery won't discharge.
When the LED is on the power consumption of the uC isn't so important.
Wouldn't it be better to switch the MCU completely off when you switch the torch off? This way the MCU power consumtion doesn't matter even when you turn off the torch.
for an easy handling I would like to use push-buttons instead of a on-off switch. I have never developed a battery application before, so I don't know to solve this without a device which is powered the whole time.
ATtiny15L (8-pin DIP/SOIC) and ATtiny26L (20-pin DIP/SOIC) are 8-bit AVR RISC microcontrollers from ATMEL usefull for small applications where space and power-saving is very much required. Both devices have 10-bit on-chip ADC and PWMs.
Be carefull with the Nitron family of devices from Motorola. If you don't want to do anything fast would be OK, but you should know that it uses an average of 4 clock cycles per instruction so these devices are quite slow.