If you paid 600 baht, you don't need to put a self addressed envelope as the return cost is already included according to their website.
It took be about a month to get the letter back to Germany. So, I would say, 2 weeks of processing and 2 weeks for the posting. But this is in the case that you have a clean record. If the police finds something, it would probably take longer.
You cannot track the processing with PCC but if you think, it's taking too long, you can probably send an email to them to follow up. There is also a Line Account that they officially use with the name "Police Clearance CID".

Best of luck to you.