Short answer
Ignore percentage of revenue rules. The number that matters is what a booked job is worth to you against what it costs to win one. If a booked job is worth six hundred dollars and costs you sixty to win, spend more. If it costs four hundred, something is wrong upstream.
Every plumber asks this, usually phrased as what should this cost. The honest answer is that the number is less important than knowing whether it is working, but that is unsatisfying, so here is a proper answer.
Why the usual advice is useless
Search this question and you will be told to spend some percentage of revenue. Those figures come from companies with marketing departments, product lines and a national footprint. They are averages across businesses that look nothing like a two van plumbing operation covering six suburbs.
Worse, a percentage rule tells you nothing about whether the money is working. You could hit the recommended figure exactly and be setting fire to all of it.
The number that actually matters
What is a booked job worth to you, and what does it cost to win one?
Work out your average job value, or better, your average margin per job. Then compare it to what you spent divided by how many jobs came in. That second number is your cost per booked job, and it is the only figure that tells you whether to spend more or less.
If a booked job is worth six hundred dollars to you and costs sixty to win, you should be spending more, not less. If it costs four hundred, something is wrong and adding budget will make it worse.
But I cannot work that out
Then that is the problem to fix first, before you touch a budget.
Most plumbers genuinely cannot answer how many phone calls their marketing produced last month, because nobody ever set up call tracking. Without it every budget conversation is guesswork dressed up as strategy, and you are relying on whether the month felt busy.
This is why tracking goes in during week one here rather than month three. It is not the exciting part, but every other decision depends on it.
What I would actually tell a plumber starting out
Do one thing properly rather than three things badly.
For most plumbers that is the free work first: finish the Google Business Profile, get the review system running, put tracking in. That is a modest monthly cost and it is the highest return work available to a plumbing business.
Once you have three months of honest numbers, you will know whether to add Google Ads for the emergency work or put more into SEO. You will be making that call on evidence rather than on a percentage somebody wrote in a blog post about a different kind of business.
One warning about ad budgets
If you do run ads, watch how your agency charges. A percentage of ad spend means every time they talk you into raising your budget they get a pay rise, and every time they cut wasted spend they take a pay cut. The incentive points the wrong way and almost nobody mentions it.
A flat fee removes that problem entirely. All the prices are on one page if you want to see what this costs.