There are several stages of baby sleep. Different babies take different lengths of time to progress from one stage to another, but they all go trough the stages:
Passive Sleep
In the newborn weeks babies sleep freely and easily. Basically they spend most of their time in the default state of asleep, waking only when a need isn't met (hunger, for example). Baby night wake for only 30 minutes of so at a time.
Learning to sleep in cycles
Out of the newborn phase, babies sleep first develops into cycles with periods of light sleeping and deep sleeping. Naps are often short because of this. To avoid over tiredness, make them frequent - say 1h awake followed by a 30-45m nap, repeated all day - 7am until you go to bed.
Sleep also becomes active, not passive. So it requires actively trying to get to sleep and stay asleep. This is the stage when sleep aids are most needed. Make sleep as easy as possible so that baby gets used to sleeping easily and often. Sucking, movement, frequent feeds, your cuddles.
Some people call this phase a regression. It isn't, it's a perminant change. Babies who already have active "getting to sleep" methods may not notice this stage. Babies who are not given enough help to get to sleep may see this stage lasting several months.
Extending Naps - Linking Sleep Cycles
At some point babies learn to link sleep cycles when napping. So naps get longer and less frequent.
This is the time of 2 or 3 naps a day - usually a 1-2h am, 1-2h pm and 30-60m at teatime. Eventually dropping the teatime nap. This is also the stage to establish "bedtime", since even wake-ups reduce when linking sleep cycles. So baby should stay asleep once going yo sleep at night.
As an approximation this happens 5m-10m, but they are all different.
Lunchtime Nap
The drop to a single lunchtime nap averages at 18 months old, but can be anywhere in the 12-24 month range.
Dropping Lunchtime Nap
Anywhere from 2y to 4y old usually.