After allowing some transition time to get used to the new activity, you should probably be spending at least as much time on cross-training as you were spending on running -- that is, not less than 10-12 hours per week. That assumes, however, that you weren't overtrained to begin with. Sometimes, an injury is a sign that you need some rest, not more stress. (Although, in my book, most injured runners rest too much -- and also eat too much.)
By the way, I generally did my cross-training at a much higher average level of perceived intensity than I did my running on land. The great thing about deep-water running, cross-country skiing, stationary cycling, etc., is that you can do some really brutal workouts -- that is, very high metabolic stress -- with relatively low stress to connective tissue and bones, because the peak loading forces are much lower than they are in running on hard surfaces. Whenever I returned to running on land after cross-training, I generally continued some amount of deep-water running to maintain the intensity of the overall program.