Of course it all depends on what your base is. I have always been fit and it took me 2 years to really improve and be strong. I am not sure what 'being used to it means' but after two years I could do a sprint on a Friday and an Olympic on a Sunday, and actually recover on the Saturday in between. I did two sports a day from the very beginning, even if it was only 20 minutes doing the second one. After one year I recovered fairly quickly.