The exo-waves are really meant to crush forcefields, honestly; the force fields are pretty much paper when the spirecraft are around. And actually if you have 50-100 bombers sitting outside a forcefield, it should be down in seconds. Otherwise, forcefields are basically just "perfect invincibility except in extremely unlikely circumstances that don't even occur in every game." Though if you're playing fallen spire, you'll obviously see that waaay more.
Hmm, now that I think about it, 100 Mk. III bombers should be able to take out a Mk. III ff in 2 or 3 salvos. I'll have to double check the math to be sure though. Thankfully, due to the somewhat slow firing rate of bombers, this can give you enough time to react. Now if you got 200 Mk. III bombers up near your ff, you've got a big problem on your hands.
Now that I have time, I have done the math. This is all on normal ship caps with normal game speed.
Mk. III bombers do 28,800 damage per shot. They also have a 6x structural bonus (which forcefields have). This gives them 172,800 damage per shot against forcefields.
Now in the hypothetical scenario above, there are 100 Mk. III bombers. Lets assume for the sake of simplicity that they all fire at the same time. This means a single salvo from all 100 of them against a forcefield does 17,280,000 damage. A Mk. III force field has 56,000,000 HP. This means that their needs to be 56,000,000/17,280,000 ~= 3.241 salvos from all of them. As bombers have a reload time of 12 seconds, this means 38.89 to 48 seconds 100 Mk. III bombers to destroy a Mk. III forcefield, depending on how "synchronized" their shots are.
For comparison, a Mk. II forcefield has 30,000,000 HP, which means 100 Mk. III bombers could take that out in ~=1.736 salvos, or between 20.83 to 24 seconds to take it out.
Make of this as you will.
(If you want statistics of Mk. I forcefield survivability under this situation, let me know.)
EDIT: Fixed some typos, most notably fixing where I put periods instead of commas.