Years ago a trainer told me that it takes 1/3 the time, of your layoff time, to get back to the same fitness level. This assumes that you train basically same as you did when you were fit. Based on this concept it would take you 5-6 weeks of training, at your pre-injury level, to get back. Injury recovery slows things down even more. Obviously this is a vague "rule of thumb" and nothing more than that. I've found it's worked for me.