What is the time unit of the converted Channel Impulse Response?
Because you don't have background in wireless communications, I will try to answer as simply as possible.


  What is the time unit of the converted CIR?


Just time unit. It can be second, ms, us, etc. They are convertible, aren't they?


  How much seconds are they (the 30 impulse responses) apart?


In the case that CSI-pilots are equally seperated, impulses are $1/(N_p \times \Delta p)$ second apart, where $N_p$ is the number of CSI-pilots, e.g. $N_p = 30$ in your example, and $\Delta p$ is CSI-pilot spacing in Hz.


  Also, are the impulses like the echoes of the transmitted signal?


Impulse reponse is the output if we use a Dirac pulse as input. It represents the effect of scaled-delayed echoes of the TX signal.


  The number of multi-path is not determined. So I assume that an
  impulse response does not represent a single path, since the number of
  impulse response is fixed to 30. Then how should I understand the CIR
  value with respect to the multi-paths?


The generated CIR is the interpolated version of the multipath response. For example, in the figure below, the CIR samples are green and physical paths are in other colors.


Note that if you accept CIR by using iDFT like that, you are implying that your multipath response is invariant during the acquisition time of your 30 CSI-pilots.

Some other resources if you want to learn more or to find the answers by yourself if I have missed something. :)

Multipath fading channel visualization in MATLAB

Statistical Characteristics of Multipath Channels