A sleeper cell is likely to do something dangerous on a rather short time scale, such as weeks, months, or perhaps a year or two. This is imminent in a much stronger sense than AI, which will take at least decades. Scott Aaronson thinks it more likely to take centuries, and this may well be true, given e.g. the present state of neuroscience, which consists mainly in saying things like "part of the brain A is involved in performing function B", but without giving any idea at all exactly how A is involved, and exactly how function B is performed at all.