I have WWOOFed - but many years ago. The time limit really depends on your host. They usually specify anything minimum/maximum in their terms and expectations. Go through the book/list and find a location and style of farm work that suits you. I do remember that some of the farms thought that English teaching/exchange was a suitable task to exchange for room and board. Some places only need a minimum of two nights. Depends on who and where they are located.
It is great fun - I encourage you to do it. You can split yo your time if you want to travel a bit. So a week at one place and a week at another. Make sure you do most of your work in the early mornings or late afternoons and use your lunch times for resting, sight seeing, craft etc.
I found it was good to know some sort of cooking dish that is typical of your country and you do a cooking “lesson” and then feed the family - this goes over really well. Even if you can’t communicate well, everybody loves to eat. When I did it, I showed everybody how to make lamingtons (an Australian cake made from bought sponge cake and dipped in chocolate and then coconut). The kids all helped out with the dipping into the chocolate and coconut and it was a fun afternoon.