The default is 500 characters. Some compatible implementations of ActivityPub have longer limits. Mastodon hides longer posts from those instances behind a link

The default is 500 characters. Some compatible implementations of ActivityPub have longer limits. Mastodon hides longer posts from those instances behind a link to open the toot[1] and show the whole thing. [1] Instances choose how to render things delivered over ActivityPub. So (for example) Mastodon would render a write.as post as a toot, while write.as might render replies to that toot as comments on a blog post.