The textbook answer is a percentage of revenue. That's not much use when you're a small business deciding between marketing and a new van.
Start from what a customer is worth
Work out roughly what an average customer is worth to you over a year, and how many more you could genuinely handle. If a customer is worth £800 and you could take on ten more a month, a £700 monthly marketing budget is not a big risk.
If you can't take on more work, don't spend on marketing yet. Fix the capacity first.
Split it sensibly
Most small businesses do best splitting budget between someone to do the work and money to put behind it. A retainer around £595 to £1,295 a month covers planning, content and management. Media spend on top usually starts around £300 to £600 a month.
Spending everything on ads with nothing decent to advertise is the most common way to waste a budget.
Judge it over months, not weeks
Give it three months before drawing conclusions, and track the boring things: enquiries, calls, bookings, quotes sent. Not reach.