Their summoning isn't as strong as ours, but their units are stronger. Which kinda makes up for it. Most Tzeentch armies you will encounter at an event will be a change host. They normally bring 2-4 endless spells. In my experience (unless they play against Seraphon or another army which doesn't cast ) they will probably be able to summon something on their first turn if they go second. And then roughly every 1.5 turns after that, depending on turn sequence, if not every turn.