When you're trying to get better at something, you need to expose yourself to challenges outside of what you're currently capable of. Without those challenges, you never get to elevate your skills.
But how much more challenging should they be? And should you constantly challenge yourself?
In the book Effortless, Greg Mekeown suggests that the challenges you undertake should be just outside of your current abilities, but this shouldn't be constant. Instead, you should push yourself, and then come back down to a more comfortable level.
Stress, then rest. Rinse and repeat.
How often you move between these stress and rest cycles is largely up to you and the activities you perform, but the cycle needs to happen. Too much stress and you'll burn out. Too much rest and you'll never grow. While a cycle of every few days may work for some activities, a cycle of every few weeks may be more appropriate for others.
The trick is to find the happy middle ground.