Question about Leitner box. How many cards to study from levels other than 1?

I realise I'm answering this three years after it was posted, but basically you should be flexible. Start doing it and calibrate both the number of boxes and the length of cycle for each box towards what works best. You've started in a bit of an unusual fashion, by making 1500 cards at the beginning. At least that is so if, as I suspect, part of the reason you are using a Leitner box is so that you can write down the answers when reviewing, which is great when learning a language and indeed when learning many other things too. But it's OK - that's what you've done and they are what you wish to remember. I would still advise adding in a number of new cards every day, though, although of course you might start doing that only after you've got into the groove if that's what works best for you. Good luck!

When using a Leitner system, you need to make some choices:

1. The first choice is to decide how many new cards to learn per day. It could be 10, it could be 40. It depends on how much time you have, since the reviews will eventually start to add up.
2. The second choice is to decide the time period for each box. If you are using three boxes, then Box 1 would be the recently learned cards / difficult cards. Box 2 could be studied every three days, for example. Box 3 could be every seven days, for example. Study everything in each box on the appropriate days. I would use more than three boxes though, since you'll need another Box for cards you know really well - you don't need to review them every seven days. You could have Box 4 as once per month, and Box 5 and once per 6-12 months.
3. Another option would be to use Leitner's original method, which involved acquiring boxes of increasing lengths and only studying cards in each box once it became full. Thus, the bigger the box, the longer the interval.