This will keep going until you CTRL-C:

yes This is stuff that I want to put into my file... >> dummy.txt

Be careful though, because you can get a tens of thousands of lines/second...

From man yes:

yes - output a string repeatedly until killed

This will keep going until you CTRL-C:

yes This is stuff that I want to put into my file... >> dummy.txt

Be careful though, because you can get a hundreds of thousands of lines/second...

From man yes:

yes - output a string repeatedly until killed