Common survey benchmarks put small-business marketing between roughly 5% and 10% of revenue — growth-mode businesses toward the top, maintenance-mode toward the bottom. Useful as a sanity band, dangerous as a formula.
The better construction is bottom-up: how many new customers do you want, times what a lead costs in your market, divided by your close rate — plus the fixed costs (site, tools). That number might be 3% of revenue or 15%, and both can be right.
Whatever the figure, fund it as a commitment, not a leftover: marketing spent only in slow months buys leads at the worst time and teaches you nothing in the good ones.