You can use a Textwatcher to determin after a textchange if the maximum amount of chars have been reached.
Try this code:
EditText yourEditText = (EditText)findViewById(R.id.editTextNew);
yourEditText.addTextChangedListener(new TextWatcher() {
@Override
public void onTextChanged(CharSequence s, int start, int before, int count) {}
@Override
public void beforeTextChanged(CharSequence s, int start, int count,int after) {}
@Override
public void afterTextChanged(Editable s) {
if(s.length() >= 200){
// Show your Button now ...
}
}
});