Purely using GNU `sed` substitution:
```sh
sed 's/\(\([^;]*;\)\{5\}\)/\1\n/g'
```

or without all the escaping backslashes using `-E` (thanks @JoL):
```sh
sed -E 's/(([^;]*;){5})/\1\n/g'
```

Example:
```sh
$ cat test.txt
a;a;a;a;a;a;a;a;a;a;a;a;a;a;a;a;a;a;a;a;a;a;a;a; etc......

$ cat test.txt | sed 's/\(\([^;]*;\)\{5\}\)/\1\n/g'
a;a;a;a;a;
a;a;a;a;a;
a;a;a;a;a;
a;a;a;a;a;
a;a;a;a; etc......
```

Explanation:

- `\([^;]*;\)`: regex capture group matching all characters up to and including a semi-colon.
- `\(\([^;]*;\)\{5\}\)`: regex capture group matching five occurrences of the above. In the `sed` command, this will be matched into `\1`.
- `s/\(\([^;]*;\)\{5\}\)/\1\n/g`: substitute (`s/`) every occurrence (`/g`) of the group of five occurrences of all characters up to and including a semicolon (`\(\([^;]*;\)\{5\}\)`) with itself (`\1`), but followed by a newline character (`\n`).