FFT Heart Rate Monitor on Arduino
1) The sampling period ($T_{s}$) is the time between each sample of the observed analog signal that is captured and stored in 'Vreal'

2) The total observation time ($T_{obs}$) is then equal to $N T{s}$ with N being the number of samples you captured

Very roughly, In order to properly monitor your signal, you need $T_s$ to be at the very least 2 times lower than the fastest observed phenomenon.
In other words, you need the sampling frequency ($F_s$) to be 2 times greater than the signal's maximum frequency (see https://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem).

Also, you need $T_{obs}$ to be great enough to capture enough heartbeats to the spectral analysis done to determine the heart pulse rate actually makes sense.

What you need to know then, is :

1) How much heartbeats you need to capture so the heartbeat can be properly determined. This will be answered by people working in the field of medicine more than people from signal processing

2) How is a heart beat represented in terms of spectral content, and how large is its spectral bandwidth so you can set up your sampling frequency high enough to be able to capture them without missing critical components