The honest answer is: it depends. Still, there's a useful guide. Many dogs do well on a rhythm of six to eight weeks. The more interesting question is why your dog might differ.
The biggest factor is the coat. Dogs whose hair keeps growing need the visit most, because without a cut it mats on the legs and belly and can block their sight. Long, silky coats need daily brushing at home. A smooth, short coat is far less demanding: once a week is enough, says the ASPCA.
Then there's the life your dog leads. One that jumps into streams, roams through bushes or romps in the snow collects more in its coat than a sofa dog. Dogs that run a lot on hard ground also wear their nails down a little themselves.
The season plays a part too. In spring, when many dogs drop their winter coat, there's more loose wool in the coat than usual.
In the end, the most reliable guide is your dog. Are the nails clicking on the floor? Does the comb catch in the coat? Is hair hanging in the eyes? Then it's time, whatever the calendar says.
Waiting too long usually costs more than it saves. If you only come once everything is matted, it's hard on the dog: grooming takes longer, and often only a short cut remains.