I noticed my breathing is very shallow. Duration of my in/out-breath is less than 2 seconds.
How long are your in/out-breaths and what is the recommended or optimal breathing pattern? The longer the better? I read somewhere that yogis should aim for the 1minute breath - 20s for each of in/out breaths and holding between in/out. Is it true?
Two seconds? Like you are panting? Are we talking everyday breathing patterns or pranayama?
SBP is a slow inhale until you reach Ajna and then a slow exhale until you reach the root. How long is each? That can change for each practitioner; the idea is not to exceed comfort level or strain to achieve a certain number and you don't hold the breath (kumbaca) at top or bottom; simply favor slow and steady while tracing the spine.
Spinal Bastrika is like panting through the nostrils while "snapping" the diaphragm. And retaining the breath, kumbaca, is supercharging any pranayama practice, therefor each of these must be approached with care and self pacing.
If you are talking everyday breathing, then two seconds is really shallow breathing and you should mindfully try to extend in-and exhales somewhat to draw more oxygen into your lungs.
There is no one proper way to preform pranayama, as each practioner has difference goals and sensitivities. Pranayama can be tailored to suit your spiritual goals.