Many contract provisions remain silent on the precise period of time within which something has to happen. The alternative for immediate or prompt action, acting upon the occurrence of a fact or acting within 30 days after a notice was received, is to allow that an action must be undertaken without undue delay. There may be many reasons for this:
the fact or event triggering an obligation (or a right) to act is itself vague;
it is not foreseeable how much time is needed in order to take the required (or allowed) action effectively and efficiently, whereas an adequate preparation is anyhow desirable;
when a triggering event occurs, there is probably no urgency to act immediately, in which case thoughtless action, merely to prevent that contractual rights lapse, should be discouraged (but also the opposite may be true);
not providing for any limitation in time creates too much uncertainty.