Power-to-hydrogen systems have the potential for long-term energy storage and diversified terminal utilization, contributing to high renewable integration in power systems. To explore the potential energy arbitrage of integrated power-to-hydrogen systems (IP2HSs) across power systems and hydrogen supply chains, we propose a long-duration robust optimization (LDRO) framework for IP2HSs considering energy, ancillary services, and hydrogen markets. A unified capacity bidding model in the ancillary service market is established for fuel cells and electrolyzers of IP2HSs to simulate the bidding strategy in typical markets. The hybrid hydrogen demand model is based on the Cournot model considering the cost-competitiveness of hydrogen in different sectors. A direct solution method combining the column-and-constraint generation algorithm and strong duality theory is proposed for the LDRO framework with quadratic terms. Uncertainty sets are constructed based on day-ahead prices in typical markets. A case study investigates the feasibility of energy arbitrage and the ability of the LDRO framework to deal with long-duration price uncertainties in 15 markets. The results highlight the advantages of hybrid hydrogen demand and the sensitivity of energy arbitrage by IP2HSs to different parameters.