The broad answer is that this will differ for everyone, and is based on factors like who you are and what your goals are in using ayahuasca. That said, most retreats that are about a week long offer 3-4 ceremonies during that time. What you can learn from ayahuasca and how most retreats operate will be based on a set of ceremonies. This gives you enough time and visits with the plant that you may find questions from the first ceremony are answered by the fourth. However, if you are financially or temporally constrained, one ceremony may be enough but it will still be only part of your process and healing isn’t finished by the end of a retreat.