How to discover the caracteristic frequency of the use of an app by someone Let's say I have an user that uses the application one time every monday exactly... That means once a week. If I do the dfft I can see a peak in this frequency

How can I analyse the general case for many users and with apparently random frequencies of use?

Is the fft the best choice?

I can use mat lab or excel to try something

This is the fft of an user of the app by day



I am assuming by it that I can say he uses the app mostly each 2.3 days (1/0.4219)... It is a coeherent conclusion? How can I get the information of all the other peaks?

EDIT
This is the signal I am making the dft
I think a Fourier based frequency spectrum analysis may not be the best (most suitable) approach in providing you the necessary and useful information about that random service usage data. 

Instead I would suggest you to use statistical data analysis methods. These methods are the primary tools that are used to analyse user behaviour by collecting usage data and computing the resulting statistics upon them.

Creating the related random variables and estimating their associated probability density functions will allow you to formulate the average quantities such as the expected number of days (or hours) between two consequetive logins, expected day of week (or hour of day) of maximum usage, minimum usage etc, and therefore enable you to make optimizations of resource utilizations based upon them.