The length of time that a baby at 5 months can do tummy time or should do tummy time is between 30 minutes to an hour.
But it depends on the baby but most 5 month olds will do tummy time for at least an hour which is good for them.
Also there's really not A set age that you have to wait to do tummy time and doctors even say that you can start tummy time the day you bring your baby home from the hospital.
Some babies may not be ready to start tummy time that early though and if not then that's okay.
My kids didn't do tummy time until 2 months of age because I felt they were too little at the time right after they were born but I've heard from doctors that you can start tummy time right after you bring the baby home.
Some people wait until the baby is 3 or 4 months of age to start tummy time and that's okay as well.
However by 3 months of age the baby should be able to do at least an hour of tummy time each day and then as the baby gets older they can usually go a bit longer.
When you first start the tummy time with the baby you should do a few minutes each day until the baby gets used to it and then can go longer.
Until at least 1 year of age your baby should also sleep on his or her back to reduce the risk of SIDS and once they are 1 year and above and they can turn over onto their side, tummy and back to their back then they can be placed on their side to sleep.
But not until then.