What everyone else said when you feel like the the party is safe. With AST I like to dot up like two mobs then, then turn it off and throw some heals, then on for some dots. Some bosses without the proper gear can be taxing for AST on mana (or maybe that was just pre-buff idk). Just need to learn to gauge tank and dps and if there is some troubled ones (tank new or dps like to stand in stuff). Over all practice and experience will help. With a stable group you will mainly be learning when you can dps in fights and less worrying about bad play making you heal a bit more.