Short answer
Profile work can move within weeks. Website rankings take roughly three months before you can judge them fairly, and keep compounding for a year or more. Anybody promising faster is either guessing or selling you ads and calling them SEO.
Every plumber wants a number and every honest answer is a range. Here is the realistic shape of it, based on what actually moves and when.
Weeks one to four: nothing visible, and that is correct
Tracking goes in. The Google Business Profile gets rebuilt properly. The first pages get written. The review system starts running.
From the outside this looks like nothing is happening, and that perception is exactly why so many plumbers get sold ads instead and told it is SEO. The work that compounds does not photograph well in week three.
The one exception is the profile. If yours was half finished, which most are, fixing categories and service areas can move you in the map within a fortnight. That is the fastest win available and it costs nothing.
Month two: the map starts to shift
Profile changes settle. Reviews start arriving steadily rather than occasionally. You may notice more calls without being able to say why.
Website pages written in month one begin to get indexed and picked up for longer, more specific searches. Not the big competitive ones yet, but the ones like replace hot water cylinder plus your suburb.
This is where honest reporting earns its place, because the movement is real but small, and without numbers it feels like nothing.
Month three: the first fair judgement
Three months is roughly when you have enough data to tell a trend from noise.
You should be able to see whether calls are up on your baseline, whether they are coming from the map or the website, and whether the pages written are earning anything. If tracking went in during week one you can answer that properly. If it did not, you are back to guessing.
If nothing at all has moved by month three, something is wrong and it needs saying rather than explaining away.
Months four to twelve: the part that compounds
This is where SEO stops feeling slow. Pages written in month one have had time to earn rankings. Reviews have built up. Each new suburb or service page starts from a stronger position than the last one did.
The work does not get harder, but the results get bigger, which is the opposite of ads where the results stop the day you stop paying.
What makes it faster or slower
Faster: an existing site with some age on it, a town with little competition, a profile that was badly neglected and therefore has easy wins in it, and a steady flow of reviews.
Slower: a brand new domain, a competitive city, a slow website, and no reviews to speak of.
If you are starting from nothing in a competitive city, the honest advice is to run Google Ads alongside for the first few months so the phone rings while the slower work builds. That is not a cop out, it is just matching the tool to the timescale.
The promise to be suspicious of
Anybody offering first page in thirty days. It is either meaningless, because they are picking a search nobody makes, or it is ads being sold as SEO.
The real answer is that the profile can move in weeks, the website takes about three months to judge and a year to really pay, and anybody being straight with you will tell you that before you sign rather than in month four.