You can apply for an extension no more than four months before your current permit will expire. You must apply before your current permit expires.

If you apply too late
If you apply for too late, i.e. after your residence permit has expired, you will not be able to work while you wait for a decision. You also risk losing certain social benefits.
Moreover, if you apply too late, there is a risk your application will be rejected because you will have been in Sweden without a valid residence permit.

If you apply for a permanent residence permit at the same time
The Swedish Migration Agency recommends that you submit your application no more than one month before your current permit expires. This is because we can only make a decision regarding your application for a permanent residence permit after your current permit has expired.