Como dizer "É a N vez que vejo..." em inglês

Suggestion: "It's been the third time I've seen a post of yours in my news feed"

It's worth noticing that "a post of yours" suggest that two or more posts were sent out to the feed (and not necessarily would have to be a news feed).

It's the umpteenth time I watch something you posted on my feed.

Ooops! I got tripped by the "N" time, and thought you had meant something else. Fortunately I have read it again. At first, I thought you meant "enésima vez"...

It's the third time I watch your video posted on my feed (your video, streaming, for isntance -to "post" could be "I read" or "watch" if it a video posted.)
If you want a generic mention and/or the three posts could be different things: "it's the third time I read or watch things sent by you (i.e.: today)."

It's the third time I watch that video you sent to my feed/...that video that you posted to my feed.

I thought of this one RedSea, but the exact words didn't come to me. Well done!