Binaries that are in "Pending Developer Release" wont expire, but you can expect an email from Apple if it stays in pending release for 30 days.
Just don't leave it so long that your certificate expires and should be fine.
I'm planning on submitting a build, with the release being set to "Pending Developer Release". There are some outstanding dependencies that I'm waiting on before the build can go live in the App Store. That being said, once an app is approved and sitting in the "Pending Developer Release" state, is there any kind of expiration on that?
My concern is that I might be ready to make the build live on day 31 or 61 (for instance) from it being in "Pending Developer Release", but then unexpectedly the build is no longer valid and instead of going live, I'd have to make another new build and go through the approval process again.
Binaries that are in "Pending Developer Release" wont expire, but you can expect an email from Apple if it stays in pending release for 30 days.
Just don't leave it so long that your certificate expires and should be fine.
Even developer certificate expiry shouldn't matter; once an app is signed by Apple for App Store release the developer certificate doesn't matter.