502 is an ok number and the actual number doesn’t matter so much as how you feel. Some guys need to get up over 700 or 800 to feel as well as they want, for other 502 is fine.
Testosterone replacement is just that replacing what the body is not producing and it is meant to be longterm. Watch out for fertility issues if you plan on having kids as T can decrease sperm quality. Clomid or HCG can be used to protect the testicle and sperm quality. It is important not to forget the estrogen as that can increase and needs to be checked. Most men need arimidex to balance the estrogen.
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.