YOU SAID:
i have been studying frogs for quite some time now
INTO JAPANESE
私はカエルをかなり長い間勉強しています
BACK INTO ENGLISH
I have studied frogs for quite a long time
INTO JAPANESE
私はかなり長い間カエルを勉強してきました
BACK INTO ENGLISH
I have studied frogs for quite a long time
That didn't even make that much sense in English.