The one I have is "Woly" Protector 3x3 at the moment.
http://www.woly.com/en/product.....roducts_pi...
I think our temperature range would be similar to you, possibly a little less extreme, but we get more rain where I live. Summer often around 25 degrees C, winter down to 0 degrees sometimes, a few frosts.
I am not a very careful person but I have looked after these boots by spraying them (about) once a month during the season (April-October could be boots season here, and since these are light I wear them a bit more than dark ones in spring and autumn). I also have a small brush that I occasionally use.