In NinjaTrader 8 you can define the hours you want the bot to run.
If you go to tools then Trading Hours you will open to the Trading Hours screen.
At the bottom left click add.
Change the Name under general to TT Trade Hours.
Change the hours under the sessions tab to the hours you want the bot to run.
At the bottom click Apply.
Then go to the strategy on the chart and under the time frame settings, go to Trading Hours and select the Trading hours you just created (TT Trade Hours)
Click ok and now the AI will only take trades during those hours.