The person who can offer one hour still has something worth organizing around.

Help gets sorted into people who can commit and people who cannot, and the second group quietly stops being asked. An hour is not nothing, though, especially if the work has been cut into pieces an hour can finish. The limit is usually in how the task was designed, not in what the person has to give.

If you need help with something, build a version that fits an hour and offer that specifically. If you are the one with only an hour, say so plainly instead of declining, because a small definite yes is easier to use than an apology.