Find symbol duration from symbol rate
The symbol duration $T_p$ is related to the symbol rate $R_s$ by the simple relationship $$T_p = \frac{1}{R_p}$$
In the case of BPSK, $R_p$ is equal to the bit rate $R_b$, and $$T_p = \frac{1}{R_b}$$
For constellations with $M$ elements, each symbol encodes $k = \log_2(M)$ bits, the symbol rate is $R_p = R_b / k$, and the symbol duration is $$T_p = \frac{1}{R_p} = \frac{k}{R_b}$$