Your billing starts on the day your pack is activated.
The validity period for all packs except Call N Trade Pack is 30 days. The validity period for Call N Trade is 15 days.
Billing cycle for all packs is 1st of every month while it is 1st and 16th of every month.
Your subscription will be active as long as your SMP ledger / Trading has sufficient funds to debit the monthly billing amount.