We have three routes from Jaipur to Ajmer. The best route is via national highway 48, which is the fastest. The distance between the places by this route is 135 kms. You should reach your destination in 3 hours.
The best time to visit Ajmer and Rajasthan, in general, is October to March when the temperature is pleasant. Winters(October to March): Winters are absolutely delightful weather in the Rajwadi land(land of the kings) of Rajasthan. November is the best month to visit Ajmer and Pushkar. It is also the time for the annual Pushkar Mela. Summers(April to June): If you want to visit Dargah for the during the annual Urs of Khwaja Moinuddin Chisti then May is your month. Ajmer is not the best place to visit in summers as the temperature can rise up to 48-degree centigrade. Monsoon(July to September): Monsoons are a welcome relief as the city is covered in lush greenery. It is the time when you can get great bargains on hotels as the holiday season is not quite there. Do carry an umbrella as rains may play a spoilsport.
The second route is along the NH48 like the regular roadmap; however, it bifurcates at Kishangarh. This route is around 151 km and takes roughly 3 and a half hour. The other alternate route is about 155 kms which also takes about 3 and a half hour.
Western India has some of the finest highways in the country. Jaipur to Ajmer route is no exception to this. A one-way cab with an experienced driver assures a safe and comfortable journey.