The answer varies based the types of prompts, how many sessions you are giving your participants a day, how many days you are wanting them to respond to this session, and how well your participants are incentivized.
A good rule of thumb is to give somewhere between 1-12 prompts, obviously, depending upon your aims. However, as you add prompts per session, you need to:
- decrease the time each question will take,
- decrease sessions per day,
- decrease how many days Users will interact with the session, and
- increase incentives
For example: if we were going to give 12 prompts in one session repeatedly, the questions would be questions that don't take long to answer (e.g., range slider, multiple choice), and we would have participants respond only 2-3 times a day, for 3-4 days. We would also incentivize our participants well; for instance, with a $25-$35 incentive.