Background: Health care systems aim to involve as much informal care as possible and dementia patients prefer to stay home as long as they can. In this context, perseverance time (Pt)—the period that the informal carer indicates to be able to maintain current care if the situation remains stable—is an important concept. Objective: The aim of this study was to introduce the concept Pt and validate it in a sample of informal carers of dementia patients living at home. Methods: Data were collected from 223 informal carers of dementia patients. Convergent validity was assessed by looking at associations of Pt with validated instruments for measuring subjective burden (CSI, CarerQol-7D, and SRB) and happiness (CarerQol-VAS). Content validity was evaluated by performing multivariate correlations between Pt and characteristics of dementia patients, informal carers, and care situations. The Medical Ethics Committee of Utrecht MC advised positively about the study protocol. Results: Correlation coefficients between Pt and the measures of burden CSI, SRB, and CarerQol-VAS were −0.46, −0.63, and 0.23 (p < 0.01), respectively. Health of dementia patient, informal carer living apart from the patient, and male gender of caregiver were positively associated with Pt; need for supervision, intensity of informal care provision, and reductions in working hours and hobbies in order to be able to provide care were negatively associated. Conclusions: Pt is helpful in monitoring need for support and planning the transition of care from home to nursing home. This study provides a first indication of its validity, but replication is necessary.