The volume key is generated using different secure random generators with additional hashing via PBKDF2. The default key length is 256 bits. Depending on the platform, the following secure random generators are used:
Windows: .NET RNGCryptoServiceProvider
Mac OS X: OpenSSL RAND_bytes
iOS: Cocoa SecRandomCopyBytes
Android: Java SecureRandom
The volume key is very strong - the strength really depends on the password you choose to secure this volume key because user passwords generally have far less entropy than a randomly generated 256 bits key. If an attacker gets access to your BoxCryptor Folder, he will most likely try to guess/crack/brute force etc. your password in order to decrypt the volume key and then decrypt your files.
More information on how to choose good passwords and password strength in generall can be found on Wikipedia: