If you look up research on this topic (Ebbinghaus, curve of forgetting), you'll see that forgetting varies. We might remember everything a day later - or have forgotten everything. This is something that varies according to learning method, person, stress, sleep, etc.
It's certain that you are going to forget everything unless you review it. Ideally, you learn something new and review it a few minutes later (look up the schedule for this if you're interested). A day later, review what you've learnt.
The crucial time would be to review something before you are at the point where you would forget it and need to "start over" learning it again.
If you're learning a language, you need to learn a lot of words in a short time. It just is absolutely no fun to keep repeating "banana" and "tomato" all over, so you need to learn a lot in order to reach a level where it is fun to use it.
Until you reach an intermediate level, learning a little every day to cover more ground seems sensible. Spaced repetition systems can help you keep track of your learning and make sure you learn the right thing every day. "Studying" is also a pretty loose term - are you sitting down with grammar book? Are you learning vocab? Are you making conversation, learning through making lots of mistakes?
One of the possible answers to this is that being really really awesome at something is having put in 10'000 hours of work, doing it. If you do it every day, you'll reach that level earlier than if you just do it every other day.
I know that I learnt fluent German so fast because I challenged myself every day, having to communicate in it in my every day life.