How loud is fortissimo and how soft is pianissimo ?
It depends on what you play. If you're playing long notes in a chord in the orchestra, you probably shouldn't play full volume or you might cover some more important parts. Also if you're playing a solo, you want to play a bit more even if it's piano. There isn't a specific number of decibel for each dynamic.
Playing extremely loud will wear you out. That's why you need to pace yourself and save your loudest volume for the climax of the piece. If you play your maximum volume all the time, it can be just as boring as if you were playing "mezzo something" the whole time. On the other end playing super soft can be very uncomfortable. It is very difficult to play with almost no air especially in the high register. So what can you do if a conductor keeps asking you to play softer and softer? It's always easy to tell people what to do but actually doing it, that's a different story!
Let's start with the soft part. First have an idea of how much volume you want for a particular passage. Then play it with that volume, not more and NOT LESS. If you try to play infinitely soft, It'll never be soft enough and you'll end up stopping your air flow and choking yourself. On the other end if you fix the volume and stick to it, you will find out that it's a lot easier to control your tone in the soft passages and you'll be able to play softer than if were trying to "disappear". At the end of the day, if the composer didn't want to hear you there, he would not have written a part!
As for loud passages, we should remember that music isn't a strong man contest. It's not about how loud you CAN play; it's about how loud you SHOULD play. Open your ears, listen to the people you play with and make your dynamics an artistic decision, not a competition! Of course, there are time when you need to use your maximum volume. In that case, go for it but choose that moment carefully; you don't want to become boring by doing the same thing all the time.
Choose a dynamic for every passage. Have an idea about how loud or soft you WANT it to be. There will always be time to talk about it if there is any disagreement. That way you become an artist making decisions rather than an athlete blowing as hard as he/she can without knowing why. You'll soon find out that most of the time, you don't need to use full volume and won't need to push yourself to the edge all the time which will take a lot of pressure off your shoulders. You'll feel better and more relaxed which will help you getting a nicer tone and make your playing a much more pleasant experience.
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.