How many times can I hit my farm account before it is teleported?

It depends on the durability of the farm and the strength of your attack.

Every city has a max durability level, which you can improve by upgrading the 'Garrison' building. When it is attacked it will loose durability. When the durability reaches 0 the city will teleport. Durability recovers over time (or immediately with diamonds).

You can see any other cities remaining durability in the world map by tapping on it. There will be a percentage bar.

When you attack check how much durability you take off each time (the stronger your APC formation the more damage you will do). Make sure you let the durability of the city recover enough so that you don't drop it to 0.