There is no single depth
Substrate depth depends on lifestyle, spider size and enclosure height. A fixed centimetre rule for every tarantula is not useful. Give the spider enough substrate for normal behaviour while keeping fall distance safe.
Terrestrials
For a terrestrial, use deep substrate is partly behavioural and partly safety equipment. It lets juveniles dig and raises the floor closer to the lid so a heavy adult cannot fall far.
Fossorials
Fossorial species need substantially more depth. Their burrow is the main retreat, and a shallow box cannot provide it. Compact the substrate enough that tunnels hold shape without becoming concrete.
Arboreals
Arboreals need less substrate depth than true fossorials, but the floor still buffers moisture, catches waste and gives slings somewhere to hide. Do not reduce it to a decorative centimetre of dry fibre.
Substrate settling
Fresh substrate can compress after watering and digging. Check the usable depth after the enclosure has settled rather than assuming the amount you first poured in remains the same.
Adjust by behaviour
If the spider repeatedly tries to dig but cannot, or a terrestrial has excessive fall height, add more. If a fossorial has already built a stable burrow, avoid tearing it apart just to meet somebody else's preferred measurement.