The weather on Rhodes is particularly good and it experiences a longer summer than some European countries, usually from around May when temperatures start to rise, until October. Even after October you can often have a nice sunny day, but temperatures may drop quite rapidly by night fall.
Summer temperatures can reach a staggering high of +30°C, with sea temperatures reaching a high of 27°C during August however, it is possible to experience days during July and August which are quite breezy so this can be a welcome relief to the very hot and humid days which are often experienced in Mediterranean climates.
Winters are usually mild but can experience heavy rainfall particularly during December and January.
So, in brief you can have a good six or seven months to visit Rhodes and enjoy some pleasant weather without packing your umbrella!