At how roughly how many entries do people usually "win" giveaways?

You can check that yourself here: https://www.steamgifts.com/stats/community/giveaways
Just scroll down till you reach "Average Entries Per Type".

The average win rate is equal to the average giveaway entry #

1/900-1000 seems to be a "decent" number from what i remeber.

I see lots of people say 1000 a lot. Naturally a lot of it depends on how many entries there are on the giveaways you join. I've been lucky enough to win 3 games in my last like 100 entries over the past 5 days, but I've been entering a lot of stuff with only 300-400 entries on them.