For most modern DSLRs, leaving the camera on and letting it enter sleep mode is usually fine. Sleep mode is designed to use very little power, and many photographers leave the camera on while actively shooting so it’s ready immediately when a moment appears.

Turning the camera fully off will generally save a bit more battery than sleep mode, and on some cameras there can still be slight drain even when switched off. Removing the battery saves the most, but that’s usually only worth considering for long storage, very long trips without charging, or extreme cold.

A practical rule: leave it on while the camera is in your hands and you may need to shoot quickly; turn it off when putting it in the bag or when you know you won’t use it for a while. That also reduces the chance of accidentally waking the camera and triggering unwanted actions.

So in normal day-to-day shooting, don’t worry much about idle time—sleep mode is usually efficient enough. If maximum battery life matters, switch it off during longer breaks.