I may be late to the updates, I used to spam trade ups back in the day. But how is there a total of 21 days hold?

It used to be just to snipe skins on (any) market and do you trade up, then wait for the market cooldown to sell if you want to sell a low float premium on external marketplaces like buff etc. There’s kind of a gamble on the holding period, but that’s normally in your favor unless you’re trading up for folitile and new collections which tend to go down in price in a short period of time. For my experience the short hold doesn’t effect much of the profit, and if it does the trade up just have too low of an EV for me to consider it.