I’d start with this rule of thumb: for a typical outdoor bench on decent soil, you want a compacted gravel base at least 4–6 inches deep. If the bench is heavy, the soil is clay or stays wet, or the area gets a lot of foot traffic, go 6–8 inches. That depth is measured after compaction, not just after dumping the stone.
A bench looks light until you realize all the weight goes through four small legs. On soft or uneven ground, those legs can punch down over time, especially after rain or freeze-thaw cycles. A proper gravel base spreads the load and lets water drain away instead of pooling under the bench.
For most DIY projects, I would dig out the area about 6–8 inches deep, lay landscape fabric, then add ¾-inch angular crushed stone in 2–3 inch layers. Compact each layer with a hand tamper or plate compactor before adding the next. Stop about 1 inch below your finished height if you are adding sand or pavers. If the bench sits directly on the gravel, leave the compacted surface slightly above the surrounding ground so the legs do not sit in a puddle.
On sandy or well-drained soil, 4 inches of compacted gravel is usually enough for a light bench. On clay or silty soil, I would not go under 6 inches, and 8 inches is safer. In freeze-thaw areas, the extra depth also reduces movement from frost heave.
One mistake people make is using round pea gravel. It shifts and never really locks together. Use angular crushed stone, often sold as paver base or ¾-inch clean stone, and compact it properly. If you do that, your bench should stay level and solid for years.