I'm not 100% sure what you are asking. If you buy it in advance, it will be active as soon as you pass through the entrance turnstiles. If you don't purchase it in advance, there is a chance that it will not be available for purchase after you enter the park.
As far as being able to use it, do you mean that there will be so many people with LL that you won't be able to book attractions? That depends on how knowledgeable and skilled you are with booking and modifying LLMP. Personally, I think LLMP at DLR is more valuable than it is at WDW - We paid for it at WDW but it only helped us with a couple of attractions at each park.