Payback < 12 months is healthy for most SaaS and subscription businesses; D2C often targets payback inside the first order.
Calculation: CAC ÷ (monthly gross profit per customer). If CAC is €120 and gross profit per customer per month is €30, payback is 4 months.
Shorter payback unlocks growth. You can reinvest faster. Voice-driven reactivation effectively shortens payback by accelerating reorder timing.