VelocityShares Daily 2x VIX ST ETN (TVIX)

[Citrati]

If you were to hold for 1 year you would lose 85-97% of your buy price. It has been that way every year since 2011 when TVIX was born.

As "Savage" said..... don't hold it for more than short term trades.
Another way to say it is to take profits quickly and build with them. Use the principle of compound addition step by step or go broke.

How to know when to sell. Trading TVIX is all about charts. If one cannot read the future consistently with them all one can do is guess. They prosper on taking guessers' money. They count on gamblers looking for the big kill and pocket ridiculous fees with a process that guarantees huge LT loses.

Course, that is just my opinion.
Do what you like.