There are two factors you should consider when thinking about how often to send a pulse question.
First, think about your employees and consider their day-to-day. How often do you think they will be happy to give feedback? (You might even want to schedule this question as one of your first pulses!). On average we find that for small- to medium-sized companies this looks to be every 1 - 2 weeks. And for larger companies every 3 - 4 weeks.
The other important factor to consider is how quickly you can make changes based on the feedback from pulse questions. You will experience a decrease in engagement if employees do not believe that change will happen as a result of their feedback.