Q: For how long does a paper continue to have the status "referee selection"?

Thanks for this question! This status essentially means that the initial screening of your paper is complete and the journal has decided that it meets the basic criteria in terms of meeting the journal scope and adherence to guidelines and can be moved to the next round – i.e. peer review. That’s a good sign for you. 

This next step can take several weeks because the journal will now look out for suitable reviewers who are experts in the field to begin the peer review. Each reviewer will have approximately 4-6 weeks for their review. Some journals also allow authors to recommend peer reviewers. If you have this option and have peer reviewers in mind, you can put in this recommendation as it will help expedite the peer review process.

I hope this helps. Good luck with your manuscript!