Payback period measures how quickly customer-level gross profit covers CAC. It's the most cash-honest unit-economics metric: a long payback means the company funds growth out of capital today and waits years to recover. Healthy SaaS payback is under 18 months; world-class is under 12; under 6 is exceptional.
Payback combines CAC and gross-margin contribution. Improving either lowers payback. Companies with weak gross margin can have unhealthy payback even with low CAC.