Their service pledge is 6 weeks from the date you submitted. Normally, 4 weeks from the issue date of the VCA no. The officer usually would review your case just a couple of days before the deadline of the service pledge, and may call you for more documents if they think your case is lack of, therefore the exact available date of the VISA may be more than 6 weeks from the submission date.
Actually, it takes only 4 working days to finish the contract verfication with the consulate, and if the agent has finished up all the paperworks, then the application should be made ready to submit to the immigration. My agent did submit my documents to immigration on the same day they collected the contract from the consulate. So if there's any delay between the contract collection date and application submission date, this means the agent has not finished up the paperworks.
What you do is you need to chase agent to make sure they finish up the paperworks prior to the date of contract collection. Once the appl goes to immigration, the maximum period is 6 weeks.
Just reviewed my case & speed up letter can help :
7/31 - agent sent all the documents to immigration
8/6 - sent the speeding letter to imm to look into the case
8/11 - received the card from imm with vca no.
8/25 - received the phone call from imm which visa approved & new one can one anytime, needs to send fax to hold the old one visa until the next come
Since the new one can't buy the air-ticket until 9/12, then the old one needs to work on that day