It really depends on how often you wish to generate a new key.  For example, lets say you have 5 friends you wish to talk to; all of them would need a copy of your PGP public key (never give anyone your private key).  In this scenario, you may choose to not even have an expiration, to alleviate constant regeneration and resharing of keys.  The function of expiration, is to invalidate the keys after a certain duration of time; for most general usage, you may want the key valid for at least a year.  Also ensure you are using a 4096 RSA AES key; anything less is impractical.