If you check the database description you’ll see that the post content column is longtext, which is:
A TEXT column with a maximum length of 4,294,967,295 or 4GB (232 − 1)
characters. The effective maximum length is less if the value contains
multibyte characters. The effective maximum length of LONGTEXT columns
also depends on the configured maximum packet size in the
client/server protocol and available memory. Each LONGTEXT value is
stored using a 4-byte length prefix that indicates the number of bytes
in the value.http://dev.mysql.com/doc/refman/5.7/en/string-type-overview.html
You will more likely run out of usable memory before the max character length becomes an issue.