Enough to buy roughly 100 clicks a month in your market — below that, you can't tell whether the campaign works or you got lucky. At a typical $5–15 cost per click for local services, that means $500–1,500/month as a realistic test floor.
The right number for you isn't a rule of thumb, it's arithmetic: your break-even cost per lead (price × margin × close rate) sets the ceiling, and your market's CPC sets how far a budget stretches. Run both numbers before picking a figure — our budget planner and break-even calculator do it in a minute.
The commonest mistake isn't spending too little; it's spreading a small budget across too many keywords and locations, so nothing gets enough data to learn from. A $500 budget on five tight keywords beats $2,000 sprayed everywhere.