How many hours daytime sleep

Hi
Was just wondering how many hours of daytime sleep a baby that is three and a half months old would usually have.
He sleeps from 7 ish until 7 at night (with a top up feed at 10).
There are quite a few days now where I am having trouble getting him to sleep during the day. I used to try and put him back down within two hours of being up or when I saw tired signs but often he just won't settle and then when he gets up is fine. Could it be that he doesn't need the sleep now that he is a bit older?
Does there become a time when they cut back on their day time sleeping and don't need as much?

Ella sleeps well at night and since about 3 months old she's gone through phases of barely sleeping during the day, just having little cat-naps (and other times, she'll sleep for 2 or more hours after her morning feed and play!).  She's happy enough during these phases - so long as I don't try to make her sleep, that is!  I guess every baby is different on the whole day-sleep issue and it doesn't matter what any of the books say on how many hours they "need" or don't need. 

Caitlin has pretty much always had a 2 hour sleep in the morning after her 8am bottle and about an hour in the afternoon (at around 2), then she has 30 mins or so around 5pm then its bedtime at 8pm (until 7.30am)

Isla doesnt take long to get tired, so is never up for much more that 1 1/4hours at a time. She then sleeps for 1 1/2 hours. Does this about 3 times a day,..then is up in the evening from around 3 ish till she goes to bed at 7pm. We had been trying to get her to go to sleep in this time,..but now just "go with the flow"! Sometimes she will go down for a bit in this time. Plunket have told me that by 4 months she "should" be up for 2 hours and down for 2, but she so cant last that long up yet.
By the look of it babies are all very different! I have to remember this when Im up in the middle of the night! We have only had one night when we have been up only twice in the night,..usually between 3-5 times a night.