Depends on the oil in fact. I have eucalyptus and melaleuca oil in the diffuser 1 hour per day and it doesnt have ill effects (they're even more friendly in fact, and the charge in the air is low). However, they never are in direct contact with the oil and I never let the diffuser plugged when they are out.
Be careful with the respiratory toxicity of some oils (geraniol, cedar and so on... are toxic to gliders). The 3 I am sure of are Eucalyptus globulus, Melaleuca alternifolia (tea tree) and orange, always organic to not have solvants remaining