I think be guided by what they prefer - a lot of babies hate being on their tummy and will cry until you move them or pick them up. Others love it. My DS hated it for about the first 6 weeks or so, so tummy time consisted of laying on my chest as I semi-reclined - he'd either snuggle in for a cuddle or would lift his head to look at me, his choice. By a bit over 2 months he was rolling to his side when put on his back, so I'd help him to his tummy as he seemed to enjoy it. Now he spends most of his day on his tummy because he rolls there himself. And then proceeds to grind along the carpet trying really hard to crawl...