I think the default is 1000, but it could be bumped up. That would be up to the admin to weigh the advantages and disadvantages.
Here's a suggestion, something I use:
When you have made a reply, before hitting the submit, check the message length. If it is too long, highlight and copy it all. Then cut off the excess and submit. Then reply, using the paste and remove what you had in the first post.
That seems obvious, but I wrote a couple that were too long and didn't really know what to do. That was suggested to me, and it works well.