My understanding is it varies, depending on the magician. The more you do it, the more the forces become a part of you. Most invocations (of forces) are done daily, like the Invocation of Jupiter, given by Jason Miller in his book Financial Sorcery, or Lady Eva’s Invocation of Divinity.
If you are invoking an actual spirit, then it is usually done once, with a finite time frame, at the end of which the spirit is dismissed. Godforms, like Odin, or Aphrodite are generally invoked rather than evoked.