Do I have to specify the retention days when I write data into Swarm or can I omit the retention days parameter?

There are no required parameters for lifepoints. In fact, you don't even need to specify a lifepoint. If you don't specify a lifepoint, the default will be to keep 'minreps' replicas, for the UUID to be deletable, and for Swarm to keep the UUID forever unless the user deletes it.

So, if you do not have a final "[]delete" lifepoint, Swarm will keep a UUID forever. You could specify that you want to retain 3 replicas forever until the user deletes the UUID with a single "[]reps=3, deletable=yes" lifepoint.

See the Swarm Guide Lifepoint Metadata Headers for more information about lifepoints.