On a non-domain joined computer, there is no lockout policy so provided it's on, the only thing they could do is try and guess your password repeatedly. If they wanted to remove the drive and access the data they would need the bitlocker recovery key. In some cases if the computer is logged in and the screen is locked, the same applies that they would need to brute-force the password, however when the computer is logged in the data in memory is decrypted so it is possible that they could power the computer off and if done fast enough they can actually dump what is in memory. This is very advanced though and fairly unlikely. Bitlocker is only data at rest encryption. It doesn't safe guard against data in-use (when you are logged in)...