u/FutureOption7493

I am curious to know when is the latest one can file an H1B extension with premium processing safely?

My understanding is that it takes 15 business days. Also how likely is an RFE? And I believe that can take 90 days. So is the answer 4 months.

Thank you!

P.S background is I'm looking to switch jobs and my H1B expires in 4.5 months.