How do you calculate CLTV without knowing how long people subscribe with some precision? Doesn't attrition rate factor into the calculation? It seems that this

How do you calculate CLTV without knowing how long people subscribe with some precision? Doesn't attrition rate factor into the calculation? It seems that this would be especially hard to determine if (a) you're a fairly new biz or (b) users stay with you for a long time.