When your sleeves are long enough, it’s time for the cuffs. But first, a word about how wide your sleeve should be at its narrowest point, before you start the cuffs. I reckon the narrowest the sleeve should be is about 6 inches, or it’s in danger of being too tight – so I advise you to check as you decrease down the sleeve, and if you’re in danger of making it narrower than 6 inches, just stop decreasing, and carry on straight to the cuff.
But of course it looks neater if you have a steady rate of decrease all the way down the sleeve, from the end of the gusset to the start of the cuff. Generally speaking, a decrease rate of 2 stitches (i.e., one either side of the seam) every 6 rows works about right.
The rule of thumb is to decrease 10% of the sleeve stitches for the cuff, and this typically gives me 108 stitches for the cuff (or 27 x 4 stitch (knit 2/purl 2) ribs – it being important to operate in multiples of 4 in order to get a whole number of ribs, as usual). Fewer stitches than that, and it can be a tight fit around the wrist.
When you have made the cuffs the correct length for your pattern – typically 3 inches – then cast off in the usual way, in pattern (casting off knit stitches with a knit stitch and purl stitches with a purl stitch) so the ribbing stays nicely pleated along the wrist.
(As with the collar, some of the books call for a couple of rows of purl stitches at the very end of the cuff, before you cast off. But I really don’t like this effect – it makes the cuff balloon out loosely just where it should be tight – and it doesn’t look right.)
When I’m knitting a gansey that’s patterned all the way down the sleeve and body, I usually make the cuffs 6 inches long, so they can be folded over to make a double-thickness cuff of 3 inches. I do this because I like the look of it – though it doesn’t look as well with a half-patterned gansey with plain lower sleeves – and because it makes for a slightly firmer grasp around the wrist. (If I’m knitting a gansey for somebody else, I usually do this anyway, because it gives them some flexibility in how long they want their sleeves, too.) But I have to say it gets pretty boring by the end – and on the final cuff, when that’s all that’s preventing you from jumping up in triumph declaring the whole thing finished, it really tries the patience.