No later than 9:00am but to be safe an 8:30am departure from downtown should give you enough time to take on all comers in lifting a brew or twisting a wrist.
If it rains (and it never rains in sunny SanDiego), all bets are off as none of know how to drive in inclement weather.
From downtown just take 1st to 5 north and get off at LaCosta Blvd go east and you are just about there.
marc