I'm afraid that it may be a matter of reading your baby to determine how much sleep is optimum for them. I think that a 12 hour night is probably optimum - not sure what Tracy said about that, been a long time since I needed to consult on babies sleep, but I might be there soon! At nearly 7 months, DD seems to need around 3 hours in the day, it splits into 2 or 3 naps, one of about 45 mins in the am, a longer one of 1.5 to 2 hours after lunch and depending on how well she slept at lunchtime she may or may not have about 30mins in the late afternoon.
On some occasions, depending on what our day is going to be like, we would have two longish naps of around 1.5hrs each. In general, at DD's age, she seems to get tired after 2-3 hours of being awake.
So far it has totally proved true that too little sleep in the day makes for rubbish sleep at night. I don't know why, you'd think that it would make em sleep better! As they get older their sleep patterns change, so it really depends on how old your LO is...