It depends. I know you were wanting a straight answer, but it's like love - different for everyone. A study by Social Bakers shows engagement dropping off after three Tweets per day <link: http://www.socialbakers.com/blog/1847-tweeting-too-much-find-out-the-ideal-tweet-frequency-for-brands>, but it depends on your followers and if Chirpsy Tweets are the only ones going out.
We recommend picking a package and trying it for a month. If interaction is low, try boosting the plan to increase visibility. If you're directly engaging with your followers a lot, then a lower plan may work best.