Testosterone can be administered parenterally , but it has more irregular prolonged absorption time and greater activity in muscle in enanthate , undecanoate , or cypionate ester form. These derivatives are hydrolyzed to release free testosterone at the site of injection; absorption rate (and thus injection schedule) varies among different esters, but medical injections are normally done anywhere between semi-weekly to once every 12 weeks. A more frequent schedule may be desirable in order to maintain a more constant level of hormone in the system.  Injectable steroids are typically administered into the muscle, not into the vein, to avoid sudden changes in the amount of the drug in the bloodstream. In addition, because estered testosterone is dissolved in oil, intravenous injection has the potential to cause a dangerous embolism (clot) in the bloodstream.
That depends a lot on what your goals are. Typically, it is considered important for the sake of your health not to take the drug continuously. You will want some time away from the drug in order to give your body the time it needs to recover. This can be invaluable time for your liver to flush out toxins and return to a healthier state before you begin again. Consider a 10-week cycle of the drug. For the last two weeks, gradually reduce the dosage to help get your body off of it before stopping completely. This will help to reduce issues regarding side effects.