I have a bathroom sink on the 2nd floor that hardly sees any action (maybe once every 3wks). The sink will drain but it takes quite some time to drain. Any suggestions on how to get this working properly again?

Sounds like there's crud that has dried up in the trap.

Try some "slow drain cleaner" to loosen it up. Follow directions, flush with LOTS of hot water afterwards.

If this doesn't work, remove trap and crud.

Let us know if this helps.