This page cannot say whether white noise is safe or good for a baby. It describes background sound near a nursery — the same mixer as every other page here, not a device built for infants — and the placement that matters most if you choose to use one.
How loud should a sound machine be in a nursery?
Quieter than the same mixer might be set for an adult room, without a figure this page can point to, since greensound has no way to measure how loud a sound reaches a particular crib from a particular speaker. The listen safely note below suggests keeping it low enough that a smoke alarm would still be clearly heard over it, and several feet from the crib, never inside or clipped to it. When in doubt, turning it down further is the safer direction, not turning it up.
How long should it run?
The timer runs for fifteen minutes a day, or with Pro for as long as you choose, from a short stretch up to a full night, and fades out gradually on its own at the end rather than cutting off. How long a sound should run near a baby, or what running one overnight does for a baby's sleep, are questions this page cannot answer. A pediatrician is the right person to ask, and is better placed to weigh that against your own household and your baby.