Product review (3 stars): This size did NOT fit Vicks V790
These 3.25" pads were recommended for the Vicks model V790 series humidifier that we have, however they need to be trimmed to fit.  Should have purchased the 2.25" size pads instead. Oh, well - live and learn.  Trimming is easy with most any scissors.
Because we run our humidifier 24/7, I would recommend changing the pads every week.  You will have mineral build up (of various amounts, depending on your water source), and it's much easier to clean up a small amount of build up weekly, than it is to wait and have to spend so much more time cleaning a bigger mess less frequently.