In normal NST, the time period for two or three accelerations that peak at 15 bpm or more above baseline is:

Definition of nonstress test (NST) currently recommended by the American College of Obstetricians and Gynecologists and the American Academy of Pediatrics: Two or more accelerations that peak at 15 bpm or more above baseline, each lasting 15 seconds or more, and all occurring within 20 minutes of beginning the test. Accelerations with or without fetal movements be accepted, and that a 40-minute or longer tracing--to account for fetal sleep cycles--should be performed before concluding that there was insufficient fetal reactivity. Although a normal number and amplitude of accelerations seems to reflect fetal well-being, "insufficient acceleration" does not invariably predict fetal compromise. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 15. Antepaum Assessment. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.