Here is my definition of a good practice session.  At the end of the session, however long it is, whether it's five minutes or an hour, ask yourself these three simple questions.

What have I learned?

What have I improved?

What have I enjoyed?

If you have three positive answers for these questions, then your practice was a great success, and you can feel relaxed that you've done all you need for the day.  

Every day is a new opportunity to increase your ability and enjoyment of playing - it doesn't just happen overnight.  It is not by great exertion but by the accumulation of many small steps that we all progress - so remember these three things, and you will surprise yourself by how fast your playing moves forward.