I think i remember seeing a post about this, but cant find it.

Anyway, what's the max pages a gump can have before spitting out an error?

I don't think it will ever spit an error, it's more like so that once a gump (with or without pages) reaches certain size (size as in the volume of all it's data), the client will crash... and I guess the critical size is different on different client versions, but in general the newer client the better stability...

Doesn't crash, but i guess its just too big like u say :p

gives the error "Error text is ->Buffer length exceeded"

If i remember it right, the maximum gump size is around 32 kB.
With a little trick you can display larger values.
For example we have a gump that has 53 pages. Simple only sent the data for the first 5 pages and build a text entry field inside the gump where you enter a page number, e.g. 46. Then resend the gump with the data of page 46-50.