When will pregnant bellies get hard?

Depends: Hardening of a pregnant belly can be related to a contraction or as innocent as a body part on the baby after movement. Be aware that a belly that becomes hard and doesn't let up and associated with lots of pain is not normal and medical help needs to sought after immediately!

Late pregnancy: When the fetus is sufficiently developed to have boney parts in the late 2nd and 3rd trimester, then the uterus will usually begin a series of Braxton Hicks contractions that will show as a "hardened" abdomen/uterus.