When is it safe stop using the baby monitor at night?

Personal Choice. It is important to know that monitors have not been shown to reduce risk of sids in babies. Turning off your monitor will have no impact on your babies sids risk. Risk reduction for sids includes breastfeeding, back sleeping in a "relatively" cool room ( 68 better than 75 degrees) without a hat. Your baby's sids risk takes a steep dive at 6 months of life.
Personal choice. Having baby in another room helps parents & baby get more rest. Otherwise well meaning checks or snoring could waken baby when the would stay asleep or settle back on their own. Having a monitor is a personal choice. Knowing what i know, i would be comfortable without one. But any parent can make their own choice and stop it when they find it more a nuscience than a friend.