There is no reliable number that applies to everyone. An estimate should follow assessment and should remain open to review.
The presentation, how long it has been present, general health, work and activity demands, goals, preferences, response to care and the need for other professional input can all change the plan.
You should be told the proposed frequency, likely duration, costs, goals and review point. The chiropractor should explain why the recommendation is proportionate and what would lead to continuing, reducing, changing or stopping care.
Ask what improvement would count, when progress will be reviewed, what you can do between appointments, what alternatives exist and what happens if the expected change does not occur.
You are not committing to indefinite treatment. You can decline a plan, choose pay-as-you-go care, request a review or stop. Ongoing supportive care is optional and should be based on an informed preference, not fear.