That will depend on haw far the seeds have to be shipped, how busy Royal Mail is, if an international order, how busy the Customs department is, how busy your mail service is and also if there are any weekend days and or holidays during shipment.
I have never had an order take longer than the number of work days Attitude says shipments will take. Keep in mind you cannot count weekend days and holidays so total overall shipment time might be longer, but the number of work days normally will not be longer than the number of days Attitude says shipment will take and if it is the fault will be that of mail services and Customs, not due to Attitude dropping the ball.