What is interesting about this question is you will already know the answer as soon as you turn on your best LED lanterns. But, in case you were wondering, as soon as you turn it on, the maximum amount of brightness is emitted. At least, the maximum amount on the current setting (high, medium, low, etc.). Here is why it works; LED lights endure a one-step process that is called electroluminescence. Basically, it is a technology that does not require the light to need time to reach its max output. Other types of lights will work on discharge technology and thus, require them to take a few minutes before they are as bright as they can be.