Anycast is widely used to improve the availability and performance of, e.g., the DNS and content delivery. To use anycast one simply announces the same prefix from multiple points of presence (PoPs). BGP then takes care of routing clients to the “nearest” PoP. While seemingly simple, managing an anycast service is not without challenges. Factors outside operator control, such as remote peering and hidden MPLS tunnels, may route clients to suboptimal PoPs in terms of latency. To successfully manage anycast, operators map catchments: the set of prefixes that reach each PoP of a service. Due to the dynamic nature of inter-domain routing, catchments change over time. While earlier work has looked at catchment stability in the short term and in a coarse-grained manner, we lack a detailed view on catchment stability over time. Understanding this is crucial for operators as it helps schedule catchment measurements and plan interventions using traffic engineering to redistribute traffic over PoPs. In this work, we put long-term catchment stability on an empirical footing. Using an industry-grade anycast testbed with 32 globally distributed PoPs, we continuously map catchments for both IPv4 and IPv6 over a 6-month period and study catchment stability at different timescales, ranging from days to weeks and months. We show catchments are very stable in the short term, with 95% of prefixes routing to the same PoP on a one-week scale, and over 99% on a day-by-day basis. We also show, however, that sudden routing changes can have a major impact on catchments. Based on our longitudinal results, we make recommendations on the frequency with which to measure catchments.