I don't know about all the various firms out there but if it's a Big 4 or mid-tier, then there should not be any problem with them signing off. Meaning they should not deny you if you have fulfilled the minimum requirements.
I worked at a Big 4 and did mine right before I left. I had friends who did it after. No one who met the requirements got denied. But sometimes it can be a lengthy wait. I heard that at my firm the person who did it only did it a couple times a year (say every 2 or 3 months). They would accumulate a stack of sign-off requests and then do them all at once. So depending on when we submitted our paperwork, we could be waiting a couple months or just a couple of days.
If you haven't left your firm yet, do it before you leave. If you are thinking about leaving, then get the process rolling ASAP. It takes more work and effort from you to do it after you are no longer with the firm. It's harder to track people down and follow up when you're not working at the same place or for them to contact you if they have questions. And it's just human nature that if something is hard, we put it off and it takes longer to do.
B - 83 Done
A - 88 Done
R - 81 Done
F - 77 Done
Ethics - Done
Attest Hours - Done
Licensed-May 2013!