every baby's milestones are different. average time span for roll over is 3-5 months and this is normal range. so dont worry. do lot of tummy time it helps the baby gain muscle strength.

Tummy time and massage helps baby in rolling over,  but never try to force a baby to roll over.  let them reach milestones on their own pace

Each baby reaches milestones in their own time. Make the baby look at black and white check pieces and also plants,  this helps them focus their vision.  Keep a plant in babies room with two different coloured leaves if possible. Also keep talking to the baby. Go very near baby's face and try and maintain eye contact.  make faces at your baby. Your baby will start responding