In 1995, James Milton and Paul Meara published the article "How periods abroad affect vocabulary growth in a foreign language", which reported on the effect of studying six months in the UK on the vocabulary size of non-native speakers of English.
Students whose native language is not English spent six months in the UK (on a Lingua or Erasmus scholarship), and their vocabulary size was tested both before and after their stay in the UK. Their average increase in vocabulary was 1326, which represented an annual increase of 2650 words. (This represents an average daily increase of 7.26 words. However, there was no follow-up test several months later, so final retention is not clear.)
There was one student whose vocabulary did not increase, and five students who scored worse in the post test. Milton and Meara write, "The growth scores varied from -900 to 3800, with an sd of 1058." So there is a lot of variability.
The researchers also found that this growth was four times as big as for students who stayed in their native country, who learn 500-600 words per year (with a much smaller standard deviation than the exchange students). However, the researcher also point out that it is not clear that the exchange students would have kept up the same growth year for a whole year (instead of the six months in the experiment). In addition, students with high initial scores progressed least; these were typically also students with high rates of vocabulary learning at home.
In an article by Rob Waring and Paul Nation from 2004 ("Second Language Reading and Incidental Vocabulary Learning"), the authors noted that (emphasis added):
Most research data we have looked at suggest that learners will learn about 3-6 words per hour of reading. If we assume that a student in school has 3-4 hours of exposure to English each week for 40 weeks a year, and one third of that is reading, this totals about 50 hours of reading per year, or vocabulary growth of between 150 to 300 words per year, not counting natural forgetting from the reading alone.
I haven't found any studies on vocabulary growth in Spanish as a foreign language, but I assume the results should not differ significantly.