I'd like to know how you would translate this into English:
"Ich moechte wissen, wie lange die Menschen noch so viel Zeit im Internet verbringen."
Why is it wie lange, but not "warum die Menschen noch so viel Zeit im Internet verbringen"?
Nope, the first one is about the future. Trust me :)
This should work as a translation:
"I'd like to know how much longer people are going to spend that much time on the internet"
both questions are perfectly correct, but ask for different things, and at different times.
wie lange noch = for how long (still) / in the future, implying it should change
warum noch = why still / in the present, implying it should have changed already
Sounds like you're just asking "I'd like to know why people spend so much time on the Internet." I think you're second sentence sounds a lot more grammatical than the first, frankly.
What I do not understand about your translation is, why you use "have been spending". The German sentence is all about the future and not the past. You wrote: " "I'd like to know how much longer people are going to spend that much time on the internet " doesn't tell us when this all started and but just implies it's continuing into the future." But that is the whole point about this sentence. It doesn't matter, if it started 5 years, 5 months or 5 seconds ago. It is only important that people are doing it right now and you wonder how long they will continue doing so. The person writing this sentence is not interested in how long people have been using the internet altogether, but only in the time-span from now to some point in the future when they will stop using the internet as much as they do now. So, I would translate it like this: "I'd like to know, how much longer people will be spending so much time on the internet."