XTEA brute force 128 bits key, 64 rounds
A 128-bit key contains 2128 possible combinations. Guessing the correct key in an exhaustive search will occur after approximately half that. This is true regardless of what cipher is used. Note that some ciphers are vulnerable to cryptanalysis which creates "shortcuts" to discovering the key. There are no known key recovery attacks against XTEA which can recover the key faster than an exhaustive search, though.