Description This paper introduces Exposure Time as a structural invariant of reasoning processes, capturing the minimum number of informational or state-update steps required before a decision can be made without violating internal consistency. Rather than treating time as an implementation detail, the framework formalizes time as a necessary resource for elimination-based reasoning. By representing reasoning trajectories through rejection coordinates and rank-limited elimination constraints, the paper shows that only a bounded number of alternatives can be invalidated simultaneously, yielding lower bounds on Exposure Time under communication, representation, and structural constraints. The formulation is intentionally model-agnostic and applies across symbolic computation, learning systems, interactive agents, and physical realizations of decision-making. It provides a unified explanation for phenomena such as search non-compressibility, instability arising from premature commitment, and the persistence requirements of long-running intelligent systems. Beyond theoretical computation, the paper discusses implications for safe decision hardware, multi-valued and undecided states, artificial general intelligence, and coherent personality over time. In this view, uncertainty and fluctuation are not deficiencies but essential components that allow independent constraints to accumulate before stable commitment. Exposure Time offers a common language for understanding why certain decisions fundamentally require time, connecting abstract reasoning limits with physical implementation and long-term intelligent behavior. Abstract Modern approaches to artificial intelligence and computation overwhelmingly prioritize speed, scale, and compression. However, both classical complexity theory and emerging AGI systems exhibit a recurring phenomenon: certain decisions cannot be made correctly without sufficient temporal exposure to intermediate states. In this paper, we formalize this phenomenon through the notion of Exposure Time, defined as the minimum number of information-update steps required for a candidate space to collapse to a stable decision. We show that Exposure Time behaves as a structural invariant of reasoning processes, independent of specific algorithms or representations. By introducing rejection coordinates and rank-limited elimination constraints, we demonstrate that only a bounded number of candidates can be invalidated simultaneously, yielding lower bounds on Exposure Time under communication and representation constraints. We then reinterpret these results in a hardware context, where Exposure Time corresponds to a minimum number of physical state transitions or clock cycles required for safe decision commitment. This perspective naturally motivates hardware architectures that enforce delayed commitment, including ternary and multi-valued logic implementations that explicitly represent undecided states. Finally, we discuss implications for AGI design, persistent personality, and hallucination prevention, arguing that failures in long-term coherence arise from structural violations of Exposure Time rather than from insufficient data or model capacity. Exposure Time thus provides a unifying framework connecting computational complexity, reasoning safety, and physical decision hardware, offering a principled foundation for persistent and auditable intelligent systems. Keywords Exposure Time, Reasoning Theory, Structural Constraints, AGI, Decision Systems License CC BY 4.0