Use a keyboard macro. (F3 and F4 mean the function keys on the keyboard.)

1. Move cursor to the beginning of the line.

2. Act once and record as macro: F3 C-u 8 C-f RET F4.

3. Play it 999 times: C-u 999 F4.

4. Be patient.

5. Voila!

You can use a keyboard macro. Generally using a keyboard macro means to record some steps for later reuse. (Find documentation about keyboard macros within Emacs by entering C-h F kmacro-start-macro-or-insert-counter <RET>.)

Recent versions of Emacs provide the function key <F3> to start a recording of a keyboard macro and <F4> to finalize the recording and also replay the last recording (if not currently recording.)

To solve you question using a keyboard macro perform the following steps.

1. Move the cursor to the beginning of the line with the 8000 characters.

2. Press <F3> (to start macro recording.)

3. Perform the action. In this case e.g. move forward eight characters (press the left arrow key eight times) and press the enter key.

4. Press <F4> (to finalize the macro recording. The macro then is recorded.)

5. Type M-999 <F4>. Concretely this (typically) means: hold down the Alt key, press the 9-key three times, release the Alt key, press <F4>. Explanation: This plays the macro 999 times. Recall that <F4> replays the previously recorded macro. One can further specify a number how often the macro will be replayed (see the Emacs documentation for "Arguments"). The M-999 stands for 999 repetitions.

6. Be patient while the macro performs.

7. See the result of the transformation!