How often cortisone injections are given varies based on the reason for the injection. This is determined on a case-by-case basis by the health care practitioner. If a single cortisone injection is curative, then further injections are unnecessary. Sometimes, a series of injections might be necessary; for example, cortisone injections for a trigger finger may be given every three weeks, to a maximum of three times in one affected finger. In other instances, such as knee osteoarthritis, a second cortisone injection may be given approximately three months after the first injection, but the injections are not generally continued on a regular basis.
McEnroe struggled to regain his form after his 1986 sabbatical. He lost three times in Grand Slam tournaments to Ivan Lendl , losing straight-set quarterfinals at both the 1987 US Open and the 1989 Australian Open and a long four-set match, played over two days, in the fourth round of the 1988 French Open. Rumors of drug abuse had begun during his second sabbatical. McEnroe denied them at the time, but acknowledged that he had used cocaine during his career in a 2000 interview that implied that the use occurred during this period, although he denied that the drug affected his play. 
In an ironic twist, the thing that causes tennis elbow (tennis) – can also help in the recovery process. Just hold a regular tennis ball held in your palm – and squeeze it as hard as you can. Initially, you will find this to be difficult – since your hand will have very little strength. As you do this more and more – you will find your grip getting stronger. This is the simplest and one of the more effective exercises that I did. I carried that ball everywhere (I didn’t care if other lunch folks saw me squeezing a tennis ball while waiting for my food..). Again – like the rest of the exercises – there is no limit to how often you do this. As much as possible!