Now different companies have different rules,
some provide a credit for 45 days,
some for 30 days and some for 60 days also.
They will warn you and call you before disconnecting your phone.
So its better to pay your bills on time and if you have opted for cancellation then within a week the process will be over once you have cleared all the unpaid bills and amounts.
If any advance is given to the company then it will refund you through a cheque which will be sent by the company may be in 30 days or so.
Keep a track it.