Innovation researchers have long used patents as a proxy for inventive activity and ideas. However, patent applications – the closest event in time widely known to the inventive activity date – always occur after the inventive activity, thus introducing an inherent time lag in the measurement. Uncertainty around this lag can prevent researchers from developing new insights about inventors, invention, and strategic HR. We estimate the likely lag distribution between activity and application by combining a large dataset of employer-employee relationships with assignee organization-inventor relationships, generating a uniquely large sample of 99,254 inventors with detailed, monthly career histories. By exploiting the common “assignment clause” of employment, we infer when the inventive steps of a patent likely took place from the employment dates of the inventors. We fit a multi-parametric model of expected lags around the start of employment as an assignee organization to estimate the lag and note that there is no apparent onboarding lag. Exploring the data further, we find limited heterogeneity in the sample between fields but observe patterns consistent with a short onboarding lag in firms that self-cite the most. The average lag is about 11 months, and we provide practical recommendations for researchers to improve estimates of the time of inventive activity.