The primary determining factors for whether you should even consider taking an offseason are the average weekly mileage you've been doing over the past several months, and what type of miles they have been. Although there are no hard and fast numbers that dictate whether you need to take a break—individual factors such as age, weight, running experience and past injury history all matter.
There are a couple things to consider. If you're injury-free and you've been running 20 easy miles per week or less, and you're taking a couple of rest days each week, then it's not really necessary to change up your training.
However, if you've just completed a big race, you typically run 40 or more miles per week, or if you've been doing a lot of high-intensity miles to build speed or power, there's a very good chance that you could benefit from an offseason. Admittedly, there's a big gap between 20 and 40 miles per week, and that's where the individual factors mentioned above come into play. The number one rule when deciding whether to scale back your training is to listen to your body. If you feel like it's time to take a breather, it probably is!