Kind of random since the game released back in 2018 and it doesn't even seem related to the upcoming Far Cry 6 but whatever.
I've never played a Far Cry game myself. I think Far Cry 4 had a trial but I missed out on it.
During the free weekend, you can play the full game solo or with a friend in co-op mode. And if you don’t already own it, you can pre-load Far Cry 5 right now. The free weekend starts August 5 at 12:01 AM PT on PS5, PS4, Xbox Series X|S, and Xbox One, 6 AM PT on PC via the Ubisoft Store, and 10 AM PT on Stadia and PC via Epic Games Store.
According to HowLongToBeat.com the main story is 18 hours.
I'll be playing it. Marvel's Avengers recently had a free trial and I actually liked it much more than the beta.