Whether it is worth what it costs.
WHAT TO MEASURE
Organic traffic, over quarters rather than months Conversions from organic traffic Revenue attributable to those conversions
Cost: time, agency fees, content production
THE CALCULATION
Compare the value of customers acquired through search against what you spent acquiring them.
Include your own time if you do the work.
THE DIFFICULTY
Attribution. Someone may find you through search, leave, return directly a week later, and phone rather than filling in a form. Simple reporting credits none of that to search.
Ask new customers how they found you, and compare that with your analytics. The gap tells you how much you are undercounting.
THE TIMEFRAME
SEO investment pays back over years, not months. Judging it after a quarter will make it look like a failure.
Content published two years ago still bringing traffic costs nothing further.
WHEN TO STOP
If two years of consistent effort produces no measurable increase in enquiries, the approach is wrong. Review what you are targeting rather than doing more of it.