I'd go for a while loop personally then cut it at the end.
While the length of str2 is less than 20, add str to str2.
Then, for good measure, we cut at the end to max 20 characters.
#!/bin/bash
str="kallel"
str2=""
while [ ${#str2} -le 20 ]
do
str2=$str2$str
done
str2=`echo $str2 | cut -c1-20`