The amount of time that a baby can use a bassinet will largely depend on how quick they grow. If you check your bassinet then it should tell you what the maximum weight is for it to be used safely. Even if your baby is not too long for the bassinet then they may still be too heavy so this is something that you will need to check.
Generally speaking, babies can sleep in a bassinet until they are about four months old. However, there will always be exceptions to this rule. If your baby is quite small then they may be able to stay in the bassinet for a longer period as they will still be able to fit in it and should not exceed the weight limit.
On the other hand, if your baby grows more quickly than expected then they may be ready for their crib earlier than four months. It can be dangerous for them to stay in the bassinet if they are heavier than the weight limit and they may not have the room to move around that they need.
There are also signs that you can look out for that might indicate that your baby is too big for the bassinet. When they can roll over by themselves it can be dangerous for them to sleep in a bassinet. There is a risk that they could roll over in the night and tip themselves out of the bassinet. This has the potential to cause serious injury to your baby and so they should be moved into a crib as soon as they are able to roll.
If your baby is sleeping through the night without waking for a feed then this is another sign that they may be ready to move into a crib. When they are not waking repeatedly during the night then there is not so much of a need for them to be in the same room as you. They will also be able to get more restful sleep in a crib than they would in a bassinet.
Many parents choose to keep their baby in the bassinet for as long as possible so they can have them in their room. As long as the bassinet is still safe to use then this is not a problem. As is the case with many aspects of your baby’s care, there is no better guide than your own judgment. If you follow the cues that your baby is giving you and ensure that you do not exceed the weight limit for the bassinet then your own instinct will tell you when your baby is ready for their crib.