Ostinato means "repeating pattern" and is used frequently in all types of music. For New Age piano players, it provides a great way to lay down the background of the piece while the right-hand improvises a melody!

For example, in the lesson "Winter Scene," we have a repeating pattern going on in the left-hand using 2 chords. And 2 chords are all that is needed to create a few minutes of music!

Interest is maintained by the right-hand melody. How long you could actually play this ostinato and maintain interest is a whole new topic but the secret to really keeping the improvisation going is to stay in the present and trust your intuition.

The music will naturally wind down when you start becoming bored or indifferent and/or no new material wants to come. Then the improvisation is over and the music has played itself out.

The good thing about using an ostinato to create with is that it's quite easy to develop a harmonic background over which you can improvise your melodies. The bad thing is that it tends to stagnate over time. That's when you either stop playing, or introduce new material - something that adds contrast.

If you wanted, you could introduce another ostinato pattern, or go for something completely different, but you'd probably want to let your intuition guide you in this. You shouldn't try to come up with material but instead, just improvise and see what comes up. A completely different, and much more effective approach!