I can’t speak for PWC specifically but it is a general rule of thumb that the quicker they get back to you the more likely an offer is to be made. As soon as a decision is made HR want to tie you up as quickly as possible.
However waiting a week does not necessarily mean you wont get an offer, it can depend on what work etc HR have going on. Generally people who get offers the following day have performed extremely well during the final AC to the point that all assessors for each component think you are a suitable candidate, and the advice from each is to offer. If you have done well in some assessments but not so well in others then there may be a meeting to discuss your performance where they effectively ‘argue’ as to whether to give you a job or not. This can take a while as there will be a number of candidates who will likely fall into this category, thats why it could take a week to inform you of their decision.
PWC might however inform all those people from an AC/interview day at the same time - many law firms do this. So they will wait till all decisions are made before informing those successful ones.
Do not give up hope! If they do reject you, definately ask as to feedback as to why they rejected you and you could always try responding that you take on board their opinion but that you really want the job, is there no way you could attend another AC to show them you are a competent and capable candidate. If they did make a ‘tough’ decision on you there might be a chance that they will allow this. I know one person who managed to do this with a medium sized accounting firm, and managed to get himself a a position after a further interview.