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Speaking quite generally no a spell will not last for ever. It will last as long as the energy feeding it does so if you keep pumping in energy the spell will keep going, if you stop it will likely stop soon after. Usually though by the time you stop putting energy into the spell it's achieved its goal and so there is no need for it to keep on going. If on the other hand you want a spell to last for a longer period of time there are methods to create external energy sources to keep powering the spell such as servitors/thought forms, blessed/enchanted objects, or just drawing on a different power source from the start such as the elements or a deity.
Taking a chant for example, which is what you stated you were doing, most chants that are going to act over a long duration in my experience (although not always) require "topping up" semi-regularly whenever the energy is running low (usually daily, weekly, occasionally monthly or in phase with a lunar cycle). All you really have to do is say the chant again and direct the energy in the same way you did before.
The McBeth curse is a superstition more than a curse. Drama is shrouded in such superstitions and most actors have some unique ones of their own before going on stage. Us drama folk aren't usually all that bright so we'll believe pretty much any superstitions you tell us and it's more the law of attraction that makes them come true. I wouldn't say it's magical in origin just a shared belief causing events to come true.
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