The only time your belly "gets hard" is when you're having a contraction, if you mean "when does it grow?" that's a completely different thing!
Honestly, anytime your belly gets hard before the 20th week, DO consult an OB/GYN doctor. It could indicate that you may be having a miscarriage. However this should not be confused with the movement of the baby, when the baby moves your belly doesn't get hard, you can feel tapping and kicking inside your belly. That's your baby either playing, experimenting, or just simply looking for a more comfortable position.
Around 26 weeks (6 months) you will start to feel hardening, tightening on your belly for a few seconds in a month's intervals (for first time mothers, subsequent mothers might get it more often and even stronger)! Your uterus is practicing what it must do when labor comes. Not to worry, that's very natural and most mothers have reported this, unless it's accompanied with discharge (bloody or watery) that is recurring (you have been to the bathroom at least twice and found lots of discharge bloody/watery). Most healthy babies now survive preterm labor after the 26th week, but there have been reports of babies surviving as little as 21 weeks.
In the last month of pregnancy, or basically 36 weeks or so, you will experience tightening on your belly more often, it may happen at least 4 times a week. This may happen sooner for some women, and for others it might not happen until the last couple of weeks... so like I mentioned it might just be your body "practicing" and getting ready for labor.
When you begin to experience pain as this happens, that's a definite sign of labor approaching, anytime within the next couple of weeks. Let your OB/GYN know about this soon, and hopefully you can make plans as to what to do when the time comes.