The short answer: more often than you think in summer, and less often than you think in winter.
Summer: every one to two weeks
From about October to April our lawns grow fast, especially after rain. A weekly or fortnightly mow keeps the grass thick and stops it going to seed. Leave it a month and you end up cutting off too much in one go, which stresses the lawn and leaves it patchy.
Winter: every three to four weeks
Growth slows right down in the cooler months. A mow every three to four weeks is usually plenty, with a tidy of the edges in between if you like things sharp.
Do not cut it too short
The most common mistake is scalping the lawn to make it last longer between mows. Cutting too low lets weeds in and burns the grass in hot weather. Taking off about a third of the height at a time is a good rule.
Edges make the difference
A lawn looks finished when the edges are crisp along paths, beds and the driveway. That is why every JKS mow includes edge trimming and a blow down of the paths and driveway.