Let's say I reserve the first page in that case, each post can have (302 maximum I guess) 300 lines each meaning we can cover 6000 entries very easily. For 40 posts the number will be 12,000. If we monitor weekly (update) then to fill up 12,000 lines we need more 9 years I guess
The post length depends, basically, on the number/type of characters used (including BBcodes)
Posts over 65535 bytes get cut off at 65535 bytes. If that makes the BBcode invalid in certain ways, then you get the INVALID BBCODE error.
Also, note that a byte is different from a character. Because SMF stores "special" characters in a particularly space-inefficient way, each Russian character uses about 6 bytes, and each line break also uses 6 bytes.