This is a very broad question that can be answered in any number of ways.
One way to look at it is to ask how intractable the problem is. This, of course, cannot be measured. All we can say here is that the problem seems to be very hard to solve. Solving it would somehow involve persuading the large majority of Afghans to support the central government. We would also have to get that government to be friendly to us. Neither of these would be simple.
Another way to look at it is in terms of cost. This is a little bit hard to quantify because there are many ways to look at which costs are really associated with this particular war. One estimate (reuters.com link) says that the US has spent $345 billion to date on this war. The same article estimates costs of between $500,000 and $1 million per soldier per year going forward.