How long should i be taking these for??

Prescribers should tell you about titrating medication (when starting go low and slow) as well as provide a timeframe for terminating the medication.  You are right to be concerned about not wanting to be on medication forever.  Also, the longer one is on a medication, there is the potential that it becomes that much more difficult to discontinue use.  Thankfully, you are asking the right questions!  I am not a Dr., but I can tell you from experience with my own Sertraline use, that you know your body and what you are feeling is cause for concern.

Based on what you are sharing it sounds like one of two issues:  (1) your dose needs to be increased or (2) Sertraline may not be the best medication for you.  There are may others that might work better with your body.  Unfortunately, you need a Dr. to help you determine which is the issue. 

Alternatively, if you think you can cope in life without the medication, consider talking with your Dr. about a VERY slow tapering off of Sertraline.  Keep in mind many people have discontinuation symptoms (sometimes referred to as withdrawal) when stopping so NEVER stop cold turkey.  Your brain and all those neurotransmitters will be so angry the serotonin train has left the station leaving you with nausea, dizziness, "brain zaps," GI issues, etc.