Leather-hard porcelain has lost some of its moisture but not as much as bone dry clay. It is firm enough to withstand some handling pressure, but there is still enough moisture left to allow a trimming tool to slice through the clay. Touch is often a better indicator than time spent drying, since drying time can vary depending on humidity and temperature.
Lightly press your finger onto a non-critical part of the piece. Soft porcelain will be easily dented or may feel slightly tacky; attempting to trim a piece that is this soft will likely cause it to warp. The porcelain should feel firm and cool to the touch, with minimal give. It should be possible to handle the piece without significantly altering its shape. Pale, chalky and fully hardened porcelain is likely approaching bone dry.
The base of the piece will often indicate the true state of the clay, especially if the rim is thin. The rim will tend to dry faster than the thicker bottom of the pot. This is important, because much of the trimming process occurs on the lower half of the pot. So it is entirely possible for the piece to appear firm, but actually be soft on the bottom. You should check a variety of spots, rather than trusting the drying time of one area.
Try using a trimming tool on the piece. If the porcelain is at an appropriate stage, the tool should cut ribbons of clay or small shavings. If the tool merely pushes the clay in front of it, the piece is too soft. If the tool skims the surface and produces brittle crumbs, the clay is too hard. Do not force the tool, regardless of the texture you encounter.
One method to learn leather-hard porcelain is to make a set of small pots of equal wall thickness. Let them dry for various amounts of time. Test them with a trimming tool and note the characteristics of the clay. Pay attention to whether the foot area will compress, whether the tool makes an even cut, and whether the rim stays stable. This method will teach you much more than trying to describe what the right texture feels like.
The moisture content should be relatively uniform before beginning the trimming process. Uneven airflow can cause the clay to dry unevenly. Be sure to turn the piece as it dries. Turning the piece during drying and using controlled coverage can reduce that difference. Do not add moisture to the clay in an effort to make it easier to trim, as this will not restore the piece evenly.
You should stop trimming once the desired shape is achieved. Do not continue trimming just to create a lighter pot. Overly thin porcelain walls and feet will be very fragile. After you finish trimming, gently run your finger along the foot and lower portion of the pot to ensure that the thickness is gradually increasing and there are no overly thick or thin spots. This simple step will provide you with valuable information that is difficult to determine by sight.