The crystallised resource will last 30 seconds from the time the spell is cast, not from when you engage with the resource. This means that the spell will wear off whether you engage with the resource in those 30 seconds or not. If another player is using the same resource node, it will deplete as normal, but the Crystallise spell's effect and timer will "pause" until the node refills. If a Fishing spot moves, the spell will move with it. Once the spell has worn off you will collect one of the resource as you would if you had not cast the spell.