So I was wondering how long Velvet and Veneer actually went to prison for
I counted up the crimes that they've supposedly done; trollnapping, troll torture, fraud, tax evasion
Yes, this is a pretty rough estimate, and was hard because the law system in MR could be very much different compared to an actual law system, and also I pretty much just compared trolls to mainly humans in this. However, assuming that V&V are older teens, possibly 16-18, this would be the estimate for their prison sentence.
Trollnapping- obviously just a copy of kidnapping, if it was "aggravated" (It involves an unlawful attack by one person upon another for the purpose of inflicting severe or aggravated bodily injury. Even making said victim scared of bodily harm alone in itself can sometimes be considered aggravated assault.) This typically leans towards the 5 year sentence from what I've seen.
Troll torture- In most states, this would be slapped down to life in prison. While this also varies in length, it is typically 15-40 years. So giving the shorter end of the commonly chosen years, this would be 15 years for Velvet and Veneer (if they are 18+)
Fraud- It can vary from 2-10 years typically. It really depends on what the fraud is related to in the crime. So for now, we'll just give them a random number and say 3 years for now.
Tax evasion- Most common sentence I could find is up to 5 years in total for tax evasion.
So all added up together, the twins would be in prison for roughly 28 years.