If you want to get totally technical minded about it, I suppose you could look at the fact it takes a Sim three Sim days to have a baby. In RL a woman is preggars for about nine months. That would make 1Day= 3 months.
Normally, I just make sure there are two Sim days between my Sim's children, to indicate a 2-3 year gap. Not accurate by any stretch to be sure, but then again making 4 Days = 1year, following the span a Sim is pregnant, can really slow down the passage of Time in a game. I play on Long as it is. I'm far too impatient to get that technical.
Hope this has helped.