Height mostly varies by class (or some "character" choices) due to the way a garment is used. A silk gown worn mostly inside can be two inches off the floor, or even train in formal situations. But if you're outside working over a fire pit, it would be filthy, ragged, or even dangerous. The general "average" that I like for late 18th century is 3-4". It stays off the ground, only gets dirty when it's raining, but it still looks very nice. I would never go more than 6" up. Even George Stubb's Hayworkers aren't more than 3" up. You aren't going to catch a hem on your shoe that high, nor would it touch the ground when bending over slightly. Be practical, it's easier to hem something another 1/2" or 1" up if it is becoming ragged than to let down.
When it comes to a matching gown/petticoat I usually make the petticoat 1/2" shorter, although I'm sure some of the mis-matched pieces I wear are an inch shorter. Much more and you might consider polonaising the gown to make it less obvious. I try to make all of my petticoats in that range, whether they have a ditto gown or not, since I don't re-enact a wide variety of classes (the fabric would be different quality any way).
Now, early 19th century follows the same rules of class and use, but tends to be longer in length. Walking gowns rise to the ankle, while many afternoon outfits even have trains. Formals are commonly seen with longer lengths in back. Riding Habits are also very long (compared to 18th century habits of normal heights).
The actual hem amount turned under is generally very small. It saves fabric that way. But, it doesn't mean you can't have an inch deep hem, I just wouldn't go much over that. Mine are usually 1/4" or 1/2" turned twice. I'm finding a slightly deeper hem common in the early 19th century, but I haven't researched that too much yet.
Now, there are two exceptions to this style of hem. Binding the bottom edge with a wool tape, often seen on quilted or wool petticoats (even more often with riding habits) allows you to have no turn up or just one turn. If you have a petticoat you want to go back-and-forth in lengths with (two wearers or two personas) that would work well since you would only have to stitch up the shorter length when needed. Or if you cut something a little too short and need that last inch.
Exception two; Silk gowns sometimes have facings. Usually a "cheaper" plain taffeta that seams to the bottom and can fold up a number of inches. Very often seen with trained gowns since it prevents a lot of dirt and wear on the expensive fabric. Easy and cheap to replace this way.
Growth tucks (or pleats) are something seen on many children's garments of the 18th century. Growing children and sharing clothing make them very practical. In the very late part of the century when gauzy dresses are coming into style (already very similar to a child's dress) we start to see them appear in adult clothing. They last through the 1860s. Usually around an 1" deep you can find examples with between 2 and 7 pleats (more later on). On a woman's gown it isn't meant to be adjustable, you even see them on trained gowns. In the 1820s they begin to pad the pleated sections, creating a precursor, perhaps, to the cage crinoline of the 1850s onward. Note on my first link, the lady on the right appears to have pleats, possibly decor on her petticoat. Whether it's an artists flare, formerly a fancier ladies, or a way to make due with second-hand and no cutting I don't know. Might be a fun challenge to seek out more of those!
Now, there is an oddity to how to hem; embroidery. Not just decorating a turned hem, but actually finishing it. Sometimes with scalloping along the edge. Not the most practical to make, but very pretty. Once again, it seems to come with the gauze trend. You do see it on under-petticoats of the 19th century as well.
Don't just look at what other people have made, take the time to do a bit of research specific to your year, fabric, and character. While museum extants show types of hems, their length is obviously skewed by the mannequins used. If you want to see good examples of fashionable lengths, fashion prints are wonderful. For more common folk of the 18th century, John Style's Dress of the People has many paintings of that class you can't easily find online.
