there is no definitive answer to this question.
It depends on the biggest variable of them all, namely the weather.
I would say that last year I took a standard type fleece, and at times I got quite cold in particular when it was being washed in the evening. The cold had a lot to do with the walking and tierdness though as opposed to the actual temperature.
I suppose it all depends on whether you are willing to carry the extra weight when it is not being worn.
I am going again on sept 4th and I will be taking a standard type fleece again. If I get cold I intend to wear my jacket over the fleece, jacket being a gortex shell lightweight.
Hoep this helps.
I didn't take a fleece on our May/June walk in wet, cold France. I took a brush cotton shirt (worn over a thin coolmax t-shirt - 100 grams). This kept me warm on chilly or windy days and kept me dry on days of light rain or drizzle weighed 200 grams and took up hardly any space in my pack.
For really wet days I also had a very lightweight (100 grams) water repellent, windproof jacket wihich I wore over the shirt + t-shirt AND a poncho too.
For really cold weather and for evenings I had a very thin low-necked jumper (200 grams - bought very cheaply in Primark) - I had planned to wear it by itself on hot evenings but we never had any so I wore it over another thin coolmax t-shirt. I found the main thing that kept me warm in the really cold, bad wet weather were my waterproof trousers.
At the last minute I put in a light fleece for my June camino and was very glad that I did. I only wore it a few times while walking but it was great in the early mornings and evenings. For September I would take one for sure.