Hi Reem,
Here's some rough estimates.
English - https://stats.wikimedia.org/
English has ~5.2 million articles, with an average of ~92 edits per article, not counting deleted edits (or deleted articles). Note that 80% of those articles are more than three years old, so they've had plenty of time to build up the 92 edits.
[The page does not explicitly say that only article edits are counted in the tables, but this is easy to confirm - https://en.wikipedia.org/wiki/
Arabic - https://stats.wikimedia.org/
Arabic has ~437k articles, ~31 edits/article - but only half of these are more than three years old, so they're on average a lot younger than the English ones.
As of July there are 3.3m edits/month in English - this is equal to an average of 0.63 edits/article/month - and 226k edits/month in Arabic, equal to 0.52 edits/article/month. July was a slow month for Arabic, and March had more than twice as many edits, 487k, across 415k articles.
These are plain averages. The distribution is going to be very skewed, so high-edit articles get most of the attention, and the other articles easily go months without attention. If we assume an 80:20 distribution - which is a wild guess but sounds plausible - then the "long tail" of 80% of articles would get 20% of the edits. In this case, a plausible average would be:
* English long tail, 4.16m articles and 660k edits/month = average of six months between each edit
* Arabic (July) long tail, 350k articles and 45k edits/month = average of seven or eight months between each edit
* Arabic (March) long tail, 332k articles and 97k edits/month = average of three and a half months between each edit