How do I know how many memory that Arduino is use after my code is running for a few time
Basically it looks like you want to figure out if you have a memory leak. Since the objects get allocated continuously you will end up having less and less free memory.

You can use the functions presented in the sister question answered very nicely by Edgar (more specifically the complex one that calculates all the memory going through the allocation chain). 

You can check the returned value every so often (few seconds/minutes depending on your needs) and if the free memory value drops continuously then you have a memory leak somewhere. This will not tell you where the leak is of course. Also this should work well while debugging the code, but would probably not be a good idea to leave it in "production" code.