As someone that competes in a sport where farmer's walks are an event, I think that the boner that the training world has recently developed over them is kinda silly.
They're a fine movement, but they tend to be overemployed as some sort of cure-all.
Got a weak grip? Do some farmers.
Need some conditioning? Do some farmers.
Core weak? Do some farmers.
Etc etc. It's akin to around 2006 or so when everyone's answer to any training problem was "Squat".
Then, on top of that, you find most trainees aren't actually doing farmer's walks, and instead are just walking around the gym with dumbbells in their hands. In reality, the dumbbell makes a poor choice for a farmer's implement due to the nature of it's design. It's constantly tying to rotate out of your hand, the pick up in insanely low, and the bells keep banging into your legs, which ultimately means using FAR less weight than would be beneficial for a farmer's walk in the first place.
All of that being said, rotating/combing farmers with some sort of front carry as conditioning work isn't a bad idea at all, just keep the intensity high. 20 meters is about as far as I'd go, with a huge emphasis on foot speed the entire time. Occasionally, load them up stupid heavy and see how far you can make it (even consider using straps), but don't do that often.