Training Break Advice

Was going to suggest getting up before sunrise, doing a short intense session so you can fuel before and after it, but it's not realistic as sunrise is so early now. Sunset is probably your best bet,   fast digesting carbs only, and jump on the indoor trainer for 45, like a short sufferfest.

Then eat again to store energy for the next day. Caffeine would help the session but it'd wreck your sleep and make it worse most likely as sunset is so late now.

Would be a hardcore fat loss month that's for sure! Put it in my mind to try it for a week or two to see how I'd fare. GL anyone who needs to do Ramadan, must take mad will power!

Not knowing all the details of Ramadhan but knowing it involves no eating during daylight hours, there's no reason this should make any real difference to fitness or training. 

A little energy basics:

You have about 400g of glycogen in yr body at any given time. Most other energy reserves are fat. 

400g glycogen is about 1200 (k)cal of exercise. 

Fat is continually used as an energy source, but when energy requirements are elevated (eg during vigorous exercise) then some / most of the energy comes from glycogen use. 

The ability to use fat as an energy source in moderately vigorous exercise is a highly trainable quality. As a simple example, I readily ride 60 miles at 20mph average without need for food during the journey. I often ride 50 miles after work averageing 20 - 23 mph (70 - 75% max HR)without eating during or prior to the ride, and I am a builder so my work is quite physical. During the evening I am unlikely to drink a full bottle during either of these rides.

While you are at rest (HR <50% max) then your energy requirements are almost 100% met by fat use, so assuming your work is non-physical it is perfectly possible to have a normal breakfast (before sunrise) not eat all day and then go for a normal training ride after work, or at the weekend put your ride off to the end of the day and ride before dinner.

The water issue is a little more difficult (I understand you're not allowed to drink water during the daylight hours either). I think the answer there is probably to ride later in the day so you can drink shortly after you finish your ride. This is largely due to comfort than any particular physiological need. Much drinking on a ride is done due to being hot, rather than liquid loss except in situations where you are sweating profusely.

In direct answer to your question:

With practise over the month of Ramadhan it is perfectly possible to go on normal training rides up to 3-4 hrs in length. The principal limiting factor is going to be how thirsty you can bear being.  

Do the most strenuous riding in the later (cooler) part of the day as then dehydration will be least issue both during and after the ride.

Very strenuous training rides (High intensity interval work, threshold level workouts) are likely to be limited to 60 - 90 minutes length due to energy availability limitations

For the sake of recovery do strenuous rides in the evening as replacing both protein and energy stores are known to be important for quicker recovery. 

Remember in all of this, though the above notes are given on sound biochemical and physiological basis, YMMV, and you may wish to check with a health professional before making lifestyle and dietary changes