Practically speaking, as long as they can convince people with control of capital (including the US gov't) that it's worth investigating.

The proper answer to your question requires information you're not going to get by asking HN, it's like asking how much time we should spend investigating fusion power. It completely depends on what leads people have, how promising the those leads look, how many "hard steps" are ahead of us, the benefit if we made it past them, etc.