Days Gone is a lot like Far Cry. It's a long game that features a large number of similar missions that you'll repeat over and over: clear out this marauder camp, go find this guy, investigate this abandoned evacuation site, burn those zombie nests, etc. It also has an inverted difficulty curve, in the sense that I found it to be pretty challenging early on but it got a lot easier after 20-30 hours when you've unlocked better weapons and some of the more useful skills. I personally enjoyed the gameplay loop, but do know that it's pretty repetitive. The current $40 price point is about right.
If you're looking for a way to kill three weeks, you could do a lot worse. This isn't a must-play like Horizon: Zero Dawn, but if "Far Cry with zombies" sounds interesting, it will probably do the job.