I can get 6 months to a year out of a brush. Woosters and Purdy's. But often only get about 3 months, as inevitably at some point at the end of a long job and I just want to get out of there, when I pack up my stuff I might wrap the brush in plastic and tell myself I'll clean it out at home. Nope, it gets forgotten and dries out.
But like ProWall says, I only let half the length of the bristles touch paint. Paint is very rarely getting up into the ferrule.
When working with acrylics, by mid day there might be too much goopy drying paint accumulating so I clean the brush and start from scratch with a fresh clean brush.
To clean i use running tap water. Smoosh the brush into the bottom of the sink with the tap water running over it. Use a wire brush to comb out the dried stuff. Then keep smooshing it back and forth under the running water till clean. Give it a quick spin with my hands, like you would spin a stick if you were trying to rub it to make a fire. No need to use an actual spinner tool. After spinning I work the bristles back into shape with my hands, instead of leaving them spread out from the spinning. Set it aside to dry overnight, or place it back in the paper cover it came in when i bought it.
But oil brushes don't last me long at all. 1 or 2 jobs usually. Cleaning them is a pain, and if wrapped in plastic and stored in a cool place, I can reuse the same brush day after day for a week or more, then just toss it out. The time it would take to clean it is worth more than the brush.
And, I'm 99% interiors only so my latex/acrylic brushes last longer