There is no "optimal amount of text per page".
As long as the content is useful to the visitors, flows, interesting, parsed correctly and obviously interesting (combinations of: text, images, videos, rich snippets, further reading etc), it can go for ever.
It's important to notice that sometimes, even one line of text is enough to make a conversion. something like: "Call now for further info" where a lead is counted over a phone call.
So, I suggest you to do what's right. If you can create a 500 quality words text that will be as descriptive and quality as possible, do it over a 2000 words text that is just repetitive and dull.
Don't forget to visualize your page - it's not enough to use text merely.
Good Luck