it likely they could brute force your passphrase
Depends on the passphrase. "Monkey" would be nano seconds. WelcomeToBoomTown would be significantly longer. BoomTownis100%redonculous would be computationally infeasible to brute force.
How many guesses can there be made per second
- Online attack scneario: 1,000 / second.
- Offline (fast) : 100,000,000,000 / second
- Massive cracking array: 100,000,000,000,000 / second.
Could hacker with the most computer power hackers use brute forc [sic] passphrase
Again, depends on the passphrase. If the passphrase was a GUID, the earth would have long been engulfed by our red giant sun, and the universe reached maximum entropy before you'd get through it with modern day computing power.
If the passphrase was on a common password list, seconds.
How strong should passphrase be to protect against it?
At least 17 characters long.
What if recovery seed would get posted on bitcoin forum and people would try to brute force it, would they be able to do it?
See all answers above.
Also, play with the password haystacks.