When should I take the colonoscopy cleansing solution?

I would recommend that you talk to your gastroenterologist to confirm that your plan is OK.

Of course you can start at 9 pm. Just be aware that you may be up later experiencing diarrhea.

That should be fine - an alternative is to take the first 3 liters starting the night before, and take the final liter about 6 hours before your procedure (so at 5am).