Complex sentence: Free neutrons decay to protons with a half-life of about 10.2 minutes, but this time-scale is longer than the first three minutes of nucleogenesis, during which time a substantial fraction of them were combined with protons into deuterium and then He-4.
Split into simpler sentences: Free neutrons decay to protons with a half-life of about 10.2 minutes, but this time-scale is longer than the first three minutes of nucleogenesis. It is during this time that a substantial fraction of them were combined with protons into deuterium and then He-4.