Marrow bones for dogs... ho long can they chew on?

so i went to the butcher shop yesterday and bought my 4 months lab a marrow bone ( she is theeting a lot, and ripping all her toys, i figure a real bone would be more usefull..) anyways.. wonderign how long should i leave her chewing the bone, how many hours a day, every day? and when should i give her a new on? every week, every 2 weeks..... thanks!! :)