The ability to kick faster and longer can be achieved with repetition and practice.  But doing the same amount of repetitions every time you train can become daunting and quite monotonous.  Repetition of doing the same kicks is just part of the territory.  No matter what, if you want your kicks to be faster, more powerful, and to out last your opponent, you will have to deal with sometimes boring drills.  It takes discipline and the best fighters out there have taken the time to endure the monotonous and experience its reward.  But there are strategies to mixing it up.
Strategies to Developing Your Kicks
 Doing a front kick one hundred times everyday for a month can lead to burnout.  Instead of doing the same amount of repetitions all at once, there are ways to break them down.

• Even – Odd Repetitions
• Timed Repetitions
• Straight Repetitions

Even-Odd Repetitions
For instance, say you want to perform fifty reps for each kick.  Break them down so that at the end of your sets, you have achieved fifty kicks.  For example, you try out even numbers, until you reach thirty, start out by doing ten, and work your way down till you reach two. 
Timed Repetitions
During timed repetitions, perform as many kicks as you can for a set time, such as thirty seconds.  Do this as many times as you want until you reach your ultimate number.  Timed drills also gives you a chance to challenge yourself.  How many kicks can you do in thirty seconds?  This can be an added benefit to kicking faster.
Straight Repetitions
There are some benefits to doing straight reps.  It will push your endurance which will help you kick much longer.  In sport karate, this is extremely beneficial.  But it will also help build your leg muscles for much faster and stronger kicks. 
Conclusion
If you seriously get tired of kicking and you have been doing enough of them, it is good to take a break from them to prevent burnout.  Try switching to developing your punching technique with the same strategies in mind.