in a hyperthetical situation, if some1 got a copy of my private key
but did not know my passphrase,
what is the probability that they would be able to decrypt an encrypted file ???
and how long would it take ???

(my passphrase is over 40 characters, and my private key uses 1024 bit encryption)

thanyou.

I don't think there are any known brute force methods for PGP/GPG private keys. The only brute force available would be on the passphrase and at 40 characters that would take a significant amount of computing power and probably years (hopefully you have a few non-alpha/numeric characters in there). It's the type of problem that large corporations and government agencies might think about, but not an ordinary user. No private entity currently has the computing power necessary for such a brute force attack. Remember that it took the EFF over 1,000 processors and 3 days to break the DES encryption and current algorithms are much more statistically improbable to break than DES.