Ich can be in a tank for a long time before your fish start getting the classic symptoms. Most of the time it is stress that weakens the fish to the point of getting ich. Stress can be caused by many things such as poor water conditions, bullying, etc. If you keep getting ich you are either not removing it completely or the condition that caused it to appear in the first place is not solved. And more than likely a combination of both these things. If you are using the classic ich meds. then it will take forever and a day to rid the tank of ich. I have said this before and will say it again ... ich meds are worthless IMO. They take too long to rid the tank of ich, all the while your fish are suffering. The quickest way to kill ich is to raise the temp to about 85 degrees and do lots of water changes. (About a 20 % a day or every other day) You also need to make sure there is not a bully in your tank that could be causing stress, and make sure your tank is not overstocked. Do some water tests and make sure your ammonia level is at 0, your nitrite level is at 0, and your nitrate is at 20 ppm or less. You pH should be the same all the time. pH fluctuations are very stressful to your fish also. Once all these things are done you should be ich free.
I wish you the best!
It reminded me to change my filter media! 
