Determining cooking time for ribs in the Texas Crutch
While I do not believe it is necessary to "crutch" ribs, it can be done, and the most popular method is called the "3-2-1 Method." (see the writeup on Steven Raichlen's Barbecue Bible site here: http://barbecuebible.com/2015/01/20/3-2-1-method-ribs/ )

The prescribed method is to cook unwrapped for three hours, two hours in foil, and one additional hour unwrapped. The method and times are merely a guideline, and are based on cooking St. Louis ribs at 225F. If you cook at a higher temperature, then adjust the times at each stage downward. If you are cooking baby back ribs, then adjust those times significantly downward. Use indirect heat the entire time, and apply sauce in the last 15 minutes of the cooking process.

Again, please be aware that these are merely guidelines. The ribs are done when they pass the tests for doneness, not at prescribed times.