I just started tummy time. How long should bub last before she starts wailing and I put her back on her back?

I started tummy time on my son when he was about 3 plus weeks for short period of times before he started wailing. He was on for about 5 to 10 minutes each time for about 3 times a day. This would gradually increase slowly whenever he allows it. Like Diana mentioned, it's probably best to do tummy time about an hour after he has had his feed otherwise he'll just vomit and leave a mess on both the mat and on himself. Sometimes if he gets cranky too quick, I'll lie down and let him lie on my chest. This way I get to rest too since he wasn't as wriggly as he is now at 7.5 months