The most perfect time to visit Rajasthan is during winter months starting from November to February last week. The weather I pretty much doable and very pleasant. There are definitely certain. Variations during early morning time to evening time. During these months mornings and evenings are colder. Afternoons are great to hangout in the winter sun and soak in all. Also, it would be preferable to carry a fleece or a jacket if you move out early or get back late. During these months visitors coming in from different parts of the world. And sometimes temperatures also drop to 1 or 0 degrees too. And since the majority of the festivals are celebrated during these months, it's better to be prepared.
Also, apart from winters, monsoon is second preferred time if you want to see lush greenery and you can go on a long drive. It actually changes the whole landscape here. The temperature in Rajasthan during monsoons range between 21 degrees to 35 degrees Celsius.
And finally, coming to summers, which usually starts from March till July last week/ August are the harshest weather. The temperatures in Rajasthan during summers range between 24 degrees to 45 degrees Celsius. Summers are not at all he ideal time to visit Rajasthan, at all. Just in case if you are out in the summer season make sure to carry extra bottle of water to keep hydrating yourself and sunscreen is a must.