Bhastrika pranayama is basically cleansing action. You are new so, adopt the speed of breathing during the pranayama to 30-50 strokes per minute or slower. Afterwards if learnt fully, the speed may increase upto 100 strokes per minute (but only after it is mastered) and there is no respiratory disorder or low stamina. You may perform repeat 3 rounds.
When you are new to the pranayama, concentrate on full inhalations and exhalations only.
After the strokes in a cycle of the pranayama are over, the breath stops automatically and let it remain suspended as long as possible. Do not exert and enjoy the cessation.
You should not be feeling exhausted while performing the pranayama; so it is the best part to do in the morning. If you feel fresh even in evening, you can do the same in evening.