Symbol of Hope is a channelled ability, restoring Mana to all nearby healers based on their missing Mana. Because it restores Mana based on the amount of Mana a healer is missing, it is ideal to use it when most of the healers are running low on Mana. As a general guide, try to find a time to use it when healers are all around roughly 20% Mana. Because it is channelled and requires you to stand still, it may take some forward planning to find an optimal time to channel the entire spell without being interrupted by encounter mechanics.