I reckon about 150 times.
In my second year of teaching, I created a worksheet for a science lesson. On that worksheet, I wrote the word 'seperate' three times.
At the end of the lesson, a colleague pointed out my error. "Er, separate has two 'As'".
So, while marking, I set to, correcting each one. 150 'separate's.
I've not spelled separate wrong since. And, I've spotted it wrong many-a-time.
Not know how to spell a word? Write it out 150 times. Worked for me!