there is a thread on istock's front page dated the 20th of August that the upload limits for non-exclusives

Base        25
Bronze     30
Silver        35
Gold         40
Diamond  45

Though this week because they got swamped by the unlimited uploads for exclusives over the weekend they have changed the limits again.

Supposedly per 24 hours though I (as a non-exclusive bronze) seem to have 10 files/48 hours supposedly it will reset to files/168 hours next week. Personally i prefer the once a week upload.