Honestly, the short answer is: check every 3 months, and tighten as soon as you feel any movement. But the real answer depends on where the bench lives and how much abuse it takes.
If it's a bench in a busy park, outside a store, or near a school, I'd check the bolts every 4 to 6 weeks. High traffic means people sit hard, kids jump on it, and the constant loading and unloading works bolts loose faster. For a residential bench on a patio or in a quiet garden, every 3 months is usually enough. I'd also do an extra check after any extreme weather, especially after winter freeze-thaw cycles or heavy storm season.
Here's the routine I use:
1. Grab the bench by the seat or armrest and try to rock it side to side and front to back. If you feel even a small click or shift, it's time.
2. Tighten all visible bolts, not just the wobbly one. When one loosens, its neighbors have usually been working loose too.
3. Use the right tool. A socket wrench or combination wrench is better than pliers. If the bolt spins without getting tight, the hole may be stripped or the insert is spinning; you'll need a washer, a longer bolt, or a thread locker.
4. Don't overtighten. You want the bolt snug against the metal or wood, but cranking it too hard can crush wood fibers or strip threads.
A few things make bolts loosen faster: benches on uneven ground, wooden slats that swell and shrink with humidity, missing lock washers, and people sliding or rocking on the bench. If you're tired of chasing the same loose bolts, add a medium-strength thread locker like blue Loctite or swap to nyloc lock nuts. You can also replace split washers with star washers for better bite.
So my practical rule: quarterly for quiet spots, monthly for public or high-traffic spots, and any time the bench has been through a big temperature swing. That keeps a street bench from wobbling without turning you into a full-time bench mechanic.