Forever. For all of time, or at least time as we know it. Does it really matter what happens after we die? I don't think so. It's all from the moment you live until the day you die, thats forever as we know it. No-one knows the true meaning of forever, we don't know anything for sure. Until time ends or until it just doesn't matter anymore. The last second of the last clock clicks the last click of it's existance.
Even what seems like eternal darkness will be met someday with light. Through light brings truth. Losing me? Pay attention. I wouldn't want to leave you in the dark.
Dreams can't last forever; infact, sometimes we don't even remember to know how long it even lasted for. Either way, the maximum lifespan for one dream is one sleep; and it can be continued at any time in any other sleep; though it will be a continuation of a former dream, and thus a new dream all together. Dreams can't last forever. You always wake up. Always.