I'm just wondering at what age should babies be able to hold their head up for a long period of time when on their tummies?

My 4.5 month DD doesn't seem to be able to hold it up for a very long time, but I'm not sure if that's normal or not??!!?

When I first put her on her tummy she holds it up pretty good for about 10 mins then she just gets really tired and it's like she's got no strength left after that time, and she keeps putting it up and down.....

She's also not really rolling yet....(she may fluke the roll occassionally, but not on purpose), is this normal by this stage?

TIA!