How long should I improvise on modes? If I stay too long on a mode, I fear that will sound repetitive. At the same time, I feel like modes are meant to be stayed on for quite a while, or else the piece will lack the modal feel.
You can improvise on a mode if that mode contains notes that suit the particular chord or chords you are playing over, and/or you can stick with a particular mode and make the flavour of that mode the 'point' of your improvisation. In the second case (if you're playing piano or are playing with an ensemble) you might want to come up with chords that conform to the mode you've chosen. If you fear you're getting repetitive, remember you still have rhythm, dynamics and tone colour to play with. How about trading or 'call and responsing' with yourself? Pretend to be two people. Check out any version of Mike Mainieri's Sara's Touch, and notice how the more modal solo sections contrast nicely with the head of the tune. Another good example of using an extended modal section to create contrast in the structure is Tal Wilkenfeld's River of Life . Notice how she sandwiches the darker phrygian-ish solos between the hypnotic head and the joyous final section. Notice also the rhythmic interest. So, in summary, you can base your entire improvisation over a chord progression on modes OR you can make one mode the whole focus of your improvisation OR you can contrast a modal improvisation section with other sections that aren't modal. No one approach is more correct than the other.
I dare to post quite a short and compact answer by quoting Duke Ellington:
“If it sounds good, it is good.”
in other words:
"if it feels right, it is right!"
I think you can play in only one mode for quite a long time, if you make it grooving or touching or just interesting. You can also play one mode over different chords, especially if you use more complex chords like with
In this example you play a d-dorian scale in the first bar over Dm in the second bar over Dm7. This is a pretty good practise to start with all modes, practice to play them over the corresponding major and minor, triads and 7th chords.