A database file is encrypted with 256bits AES and the key is the first 7 characters (from the character set 0-9, a - f, all small letter) of MD5 hash . This implies 2^28 bits (268,435,456) possible passwords. How long will take to brute force such a password?
To clarify and expand what I said in the comment:
A GPU-based cracker can go through a LOT of MD5 hash per seconds. According to this, one based on the (old) NVIDIA Geforce 8800 Ultra can compute about 200 million hashes per seconds. Since your keyspace is a bit above 2 millions entry large, it means that you can get through ALL of it in about one second and a half.
That doesn't, however, tell you the whole story: you're not trying to generate a collision here but you're trying to find the key used in an encryption scheme. This means that each of these hashes must then be tested. It is very possible - even likely - for this step to take much longer that simply generating a hash. Exactly how long depends on how the chosen algo is used exactly and at what stage you can decide if the key is valid or not.