will the loot in the clan castle ever disappear, or will the loot stay in there forever if you don't collect? I havnt tried as I don't want to lose 500k of loot
Loot stored in the clan castle will kept in the treasury of the castle, and will stay there for as long as you decline to collect it. Any loot you earn from additional wars or daily 5-star bonuses that does not fit in the clan castle storage is lost.
The only exception to this is when your main village is raided. Attackers can steal up to 3% of the loot stored in a clan castle's treasury when it's destroyed. Since normal storages allow attackers to steal 20% of what they contain, it is often best to keep resources stored in the treasury until such time as you need to use it or to free up space for more war loot.