Paid search can produce enquiries the day it starts, because you are buying placement in front of people already searching for what you sell. It also stops the day you stop paying, and everything spent produced customers rather than an asset. That is not a criticism. For a business that needs revenue this quarter it is frequently the correct choice.
Search visibility and content work on a completely different timescale. Meaningful movement takes months, typically somewhere between four and twelve depending on how contested the terms are and how established your site is. Then a page that ranks continues producing without further spend, indefinitely, which is the opposite economics. Judging that work by the timescale of advertising is the most common reason it gets abandoned at month three, which is usually just before anything would have shown.
Reviews sit somewhere between and are the fastest compounding asset available. The effect on whether somebody chooses you is immediate once the volume is there, and getting to a credible number takes a few months of asking consistently rather than a campaign. For a local business this is frequently the shortest path from effort to result.
Email produces results quickly once a list exists and produces nothing before then, which makes the honest answer about timing dependent on where you are starting. Building a list of a few hundred people takes a year of collecting addresses, and the first send to an existing list can produce enquiries the same day.
Referrals and partnerships have the longest lead time and the highest quality output. A relationship with an accountant or a complementary business rarely produces anything in the first three months and can produce steadily for years. Businesses expecting quick returns here conclude it does not work and stop attending, which is exactly the behaviour that prevents it working.
The practical consequence is that you should run more than one at different timescales, and judge each against its own. Something producing now, something accumulating for later, and the honest acceptance that the second will look like failure while it is working. A business running only the fast channel is renting its customers. One running only the slow channel may not survive to benefit.
Set the review point when you start rather than deciding as you go, because that is what prevents both errors. Write down what result would justify continuing and when you will assess it. Businesses quit at three weeks when the test needed eight, or continue for a year because stopping would mean admitting the spending was wasted.
Then measure by enquiries rather than by traffic, rankings, or impressions, because those move for reasons unrelated to whether the business is growing. The question is how many people contacted you and where they came from, and answering it requires the tracking to exist before the spending starts rather than afterward.
Be careful about judging a channel by its first month specifically, since almost everything performs badly initially and improves as it is adjusted. Advertising needs negative keywords added and pages refined. Content needs to accumulate. A channel abandoned after four weeks was abandoned before it was working properly rather than because it does not work.
Record the starting position before you begin, because without it the eventual assessment has nothing to compare against. Current enquiries per month, current traffic, current review count. That baseline takes ten minutes to capture and is impossible to reconstruct afterward.