Before OPT ends, how many months in advance is it appropriate to apply for a CPT university?

I would like to know about the timeline. How long does it usually take to receive the admission result after applying? When can the I-20 be obtained? How long does the application preparation take?

I’ve consulted with couple friends who went through this. they said that admission only takes about 1-3 days. but to keep it safe, you should apply 1 month before the semester start. I know some school even provide expedited service that can issue i-20 within 48 hours.