I say five.

Then, when that becomes too easy, 7.

Then when that becomes too easy, 10.

Then when that becomes too easy... you get my drift.

Set a reachable goal, reach it, and then stretch father! There is no upper limit.

At a summer school I went to in Yokohama we had to learn, literally, 20-30 new Kanji per day, including 2 or 3 vocabulary items to go along with them. It was super hard, but it's possible!

がんばれ！

It's obviously very hard if you have a full time job, but when I was in language school we learned about 10 kanji a day and about 30-50 total vocab to go along with those kanji.

The most efficient way is to get word lists organized by topic that list the words in descending order by frequency.

When learning verbs, I found it is crucial to also remember the particles that usually come with it because sometimes the grammar doesn't translate the same into English as you'd expect. For example: （電車）に乗る、（公園）を歩く

I think once you get past beginner's Japanese, reading texts with a dictionary beside you will teach you many more words per day than you ever thought possible.

I say a 10+ Reading text and watching japanese movies. Its everything with the japanese language. the best way to understand a new language. Just got to dive right in and be srounding my japanese other wise you will just lose everything youve been tought if you dont keep practing everyday.

whem i wanted to learn french,i would learn 20 worda a day and then at the weekend i had a review on all the words. the result was great for me. take it easy.learn with interest

5 words a day, as JapanNewbie suggested. This is an attainable goal. I am currently taking Japanese at Cal in Berkeley and we learn roughly 30 new vocab every week, in addition to the readings/writings of 200 kanji every two weeks!

However, I would also like to add that quantity isn't as important as how much you can learn AND retain. There is nothing more frustrating then thinking you have learned something only to go back two weeks later and find your mind drawing a blank. How much time you have on your hands is a major factor, as well as how much you plan to go back and review as you continue to study.

In the end, my suggestion is to pick 5 vocab a day, and instead of JUST learning the vocab, learn the kanji for that word at the same time. This way, you are killing two birds with one stone- AND keeping it all connected.

(and as Osaka-Nannin suggested, learn those particles with any verbs you study. Absolutely crucial!)