I believe a balance of 1000 PHR would give you roughly 1 stake every three days. But even then, it is on average, meaning you could easily go a week without any stakes, but you could also get 2 stakes in a single day. Over a long time it would average out to one stake every three days.
Is there any information about inputs optimizing (their size) for better PoS rewards??
The chart at the bottom here shows the average block size for winning POS blocks:
The "ideal" seems to be between 600-800 at the moment.