I only have the 6.5" silent spinners so it's not exactly the same, but hopefully I can give you some general ideas about them.
- What is the "lifespan" of a large (12-inch) Silent Spinner wheel?
Don't have one, so cannot comment.
- Does it say anywhere how long it lasts, averagely?
From reading a lot of online reviews, it seems to vary from a few weeks to a few years. I've rarely came across anyone that wrote the wheel becomes totally unusable, but rattling over time is a very common issue.
- How long have you had yours for and is it still in good shape?
I currently have four 6.5" SS all currently in use, I've had them varying from 6 months to almost 2 years. They're all still in good shape, but the oldest one is now rattling fairly loudly when Bumpy (my heaviest robo) runs on it. The rest are still fairly silent (they're all next to my bed).
- Would you call the wheel sturdy, or does something about it bother you? If so, what?
If you get a good one, I think it is pretty sturdy. But if you get a rattling one, the wheel tend to wobble around and move all over the place. This could be a problem with a syrian running on it. I think the worst part is that once the wheel started rattling, you can't really fix it (yellow axel piece in the back of the wheel).
- Did a part of your Silent Spinner ever break? If yes, what and how?
None of mine had ever break. The plastic is thick and good quality in my opinion.
- Also, how is the wheel removed from the stand for cleaning?
It's very easy to take a part for cleaning. Just unscrew the yellow knob in the middle of the wheel and the wheel comes off. When you put it back together, make sure the tiny notch in the back center of the wheel is slotted into the yellow back piece properly. It's hard to describe it, but you'll know it when you see it.
- And lastly: is it really 100% silent, or at least as silent as a wooden wheel (Karlie)?
Depends on your luck.
Mine are mostly silent except the sound of tiny feet running, but when it start to rattle it can be pretty loud.