Tummy time....I have a question for you mommies who have a little one around 3 months...my baby has…

Tummy time....I have a question for you mommies who have a little one around 3 months...my baby has been doing tummy time since she was born but now 3 months later she will still only do it for a few minutes at a time, if that long...how long do your little ones handle doing tummy time? we do it on the floor a couple time a day until she gets upset, we also lay her on our chests and do it that way, we always lay down on the floor with her with toys, and never right after she eats to avoid spit ups and upset tummy. she does hold her head up well, today was rolling on her side, and doing crawling motions with her butt in the air lol scooting forward a little...should I be at all concerned that she only does it for short times?