Anybody that has had these for a while, do they seem durable?

Yes and with proper care and maintenance they will last a long time I had my last set almost a year befor I misplaced them lol 😄

What do the different sizes 3,6,9,12 mean? If I only wanted to trim a bit of my dog’s hair off, which one would I use?

If you only want to trim a little, I'd start with the largest/highest number (ie 12mm/12 millimeter- just like for human hair trimmers), and trim going with the direction of the fur/hair growth. If you go against the hair (or against the grain) then you'll be right up against the base of the hair growth and well... you'll probably take off way more hair than you're meaning to and what's done is done. I've done that. It was kind of hilarious but did make me learn really quick of what not to do. There are also numbers on the trimmer that I'm going to call the dial that adjust the length of the blade- again, go long and work down. By starting with the longest length attachable comb going with the hair, and afterwards you're like hmmm, that's still too long, then do it again but switching out the attachable comb for the next one that is short (if you started with the 12mm, then replace the 12mm with the 9mm). You can always take a little more length off, but you can't put length back on. Clean out the trimmer as you go to keep it functioning/cutting at its best and not overheating, which can really comfortable and sometimes painful to a dog's/animal's sensitive skin, which is why it's better to use a trimmer (sometimes I see them called clippers) that are made for dogs/animals on dogs/animals because they don't heat up as quickly and are made for longer use. Those that are made for human hair trimming are not made for extended use while going over a full body. Holy moly, I got a little deep into the details but hopefully it helps. Lol!! If not, or if you want even more, ice found soooo many helpful videos on YouTube. Plus, I'd rather have someone show me instead of me having to read that massive response I just wrote. Hopefully this long response didn't make your eyes cross by the end of it, if so... I'd suggest waiting a little bit before jumping into that trim. 😉

I don't know. Seller should answer. It is very slow speed. Only one speed use...slow. Ok tho for my dogs length of fur.

How long does it take for them to get hot?

hi client. Under normal use, there will not be get hot. if working time too long.suggest pause for a period of time to continue using.can extend the service life