You can set a time limit for a specific app. When you activate it only that app can be used and only for as long as you have set the time limit. This is how you do:
First you need to activate guided access in iOS.
Go to Settings > General > Accessibility > Guided Access and activate it.
If you want to have sound reminders before the time runs out, you can choose the sound under Time Limits, but that's optional.
Next you open the app and set your time limit.
Open an app you want to restrict usage to.
Triple-press the home button and choose Guided Access in the pop-up that opens.
Tap Options under Time Limit.
Activate Time Limit and choose a time limit.
Tap Start in the upper right corner. Now only this app can be used and only for the time you have set. When the time runs out the iPad screen will be locked.
The first time you do this you will be prompted to set a pass code. This is for restricting access to the limitation settings. Triple-press home button to stop ahead of time.