When I was in Nashville, we would write 2 (or 3) a day. You have writing appointments. A 3 hour session with another writer. 2 or 3 appointments a day. That is an extreme writing situation where you are employed by a publisher to write. It's your job. 5 days a week. Minimum 10 songs per week.
When writing by myself, I'm happy when I can take a little more time and spend 2 or 3 days on one song. Even a week, (especially to tweak lyrics).
The key (at least for me) is NOT waiting for inspiration. For every song I was inspired to write, I have written 50 by just plowing through and doing the work - even when I didn't feel like it. And those ended up being some of my best songs.
"Writer's block", "inspiration" are excuses used by those who don't want to do the work. Just write. Good, bad - it doesn't matter. Just write. If you don't like the result, then re-write and make it better. Unless you're a genius, all your initial ideas will suck. Re-write. Re-write. Re-write. Take a half hour break, then re-write again. When it gives you goosebumps, then you're finished.