Singlet fission, the conversion of a singlet excitation into two triplet excitations, is a viable route to improved solar-cell efficiency. Despite active efforts to understand the singlet fission mechanism, which would aid in the rational design of new materials, a comprehensive understanding of mechanistic principles is still lacking. Here, we present the first study of singlet fission in crystalline hexacene which, together with tetracene and pentacene, enables the elucidation of mechanistic trends. We characterize the static and transient optical absorption and combine our findings with a theoretical analysis of the relevant electronic couplings and rates. We find a singlet fission time scale of 530 fs, which is orders of magnitude faster than tetracene (10-100ps) but significantly slower than pentacene (80-110 fs). We interpret this increased time scale as a multiphonon relaxation effect originating from a large exothermicity and present a microscopic theory that quantitatively reproduces the rates in the acene family.