Normal range is between 300 to about 1050ng/dL, peaks at about age 20 and declines with age, at about 1% each year from age 30 on...
Your Doctor can prescribe a dose that will get you between those ranges and TRT to maintain them by checking your blood work. If you have a good Doc, he/she will ask you how you feel on the treatment and will adjust the dose to get you to a range where you feel much better about yourself (for a lack of a better term). But they will not take you over the normal range...
A lot of different factors can cause a drop in your test levels, gear being one of the main ones... Once you introduce synthetic testosterone (which TRT is) your body will stop producing it. Thus testosterone replacement therapy is normally a lifelong process...
I've been on TRT (Test Cyp 200mg e2w) for less than a year but found it helped tremendously in the first few months but then it tapered off some. I'll have blood work done to see where my levels are and adjust from there. The closer you are to the higher side of normal levels, the better you'll feel is my opinion. But over all, I feel 100% better now that I have my test levels in the normal range and recommend TRT for anyone who may need it.