Lengthening a top down sleeve

I am knitting a top down cardigan with short sleeves and want to lengthen the sleeves. How often should I decrease. I am going from quite short to 3/4 length.

Welcome to the forum!
Take a couple of measurements.

  1. Measure near the start of the decreases around your upper sleeve and 2. measure around the end of the sleeve on your lower arm.
    With those measurements you’ll be able to calculate how many inches you need to narrow the sleeve opening. Knowing your sts per inch (stitch gauge) you can calculate how many sts to decrease.

  2. Now measure the length of the decrease portion of the sleeve.
    Knowing how many rows per inch (the row gauge) you can figure out how many rows you’ll have in the decrease portion of the sleeve.

Divide the number of rows by the sts to be decreased and you’ll know how often to decrease. Remember too that you’ll likely be decreasing 2 sts per decrease row.

Thank you for your quick response.