Flip the common question: mature service businesses typically SPEND 5–10% of revenue on marketing; what marketing should GENERATE depends on how much growth you're buying. A useful frame: new-customer revenue ÷ marketing spend should comfortably exceed 3:1 on margin. Below that, marketing is a treadmill, not an engine.
The budget audit is an afternoon: pull twelve months of marketing charges (cards, invoices, tools), bucket by channel, then set attributed customers beside each bucket. Every business that does this finds a zombie. A subscription or retainer producing nothing that nobody remembered to kill.
Make it annual: the audit-then-reallocate cycle is how budgets evolve from history ('we've always paid for that') to strategy.