How often a bike needs a proper service depends far more on how much it's ridden than on the calendar. A bike ridden daily for commuting wears through consumables — chain, brake pads, cables — much faster than one ridden occasionally on weekends.
As a rough guide: a daily commuter benefits from a full check every 3-4 months, a weekend-only rider can usually stretch that to 6-12 months, and any bike that's been sitting unused for a season is worth a check before the first ride back — tyres lose pressure, cables can seize, and brake pads harden with disuse.
Between full services, the quick checks (tyre pressure, brake feel, chain lubrication) take a few minutes and catch most problems long before they need a proper workshop visit.