Complex sentence: Following his accession he spent very little time, perhaps as little as six months, in England; most of his life as king was spent on Crusade, in captivity, or in actively defending his lands in France.
Split into simpler sentences: Following his accession, he spent very little time, perhaps as little as six months, in England. Most of his life as king was spent on Crusade, in captivity, or actively defending his lands in France.