Within 1-60 days you have to get a signed i 20 with start date that's in upcoming 90 days..
This includes the day you get your new I20, don't aim for 60 days, any issues you loose status. Aim for 45 days. Give 15 days for delays of whatever.

You classes have to start within 90 days of your signed I20. Again dont aim for 90. This will depend on when the actual semester starts at the school.
Beyond 91 days you will loose F1 status.

@maxpayne OPT ends on 15th Oct and advisor said we can issue the I-20/update on December 11th, December because its the same school, I am applying for another program. And the session will start from 11th March, it will actually cross 90 days (they said it's fine which was weird) unless I request to issue on the 14th of December. I didn't want to start too early, I needed some time. Other universities start almost 3 months earlier or way late.

Does anyone have any idea how is University of Cumberlands? I have been trying to reach them. I heard their tuition fee is $3,600 per semester but the first semester, they charge $500 more ? I heard it's a good school. Please let me know any information about the university or any other good school where I can handle full time work and school, easy to get admission, not too extensive. Guys help me out with this.

This is very strange that every universities are giving me different answers. One said it is based on my OPT not I-20. I got more info below:

Please see 8 CFR 214.2(f)(8)(i) on F-1 student regulations website.

(8) School transfer.

8 CFR 214.2(f)(8)(i)

(i) A student who is maintaining status may transfer to another Service approved school by following the notification procedure prescribed in paragraph (f)(8)(ii) of this section. However, an F-1 student is not permitted to remain in the United States when transferring between schools or programs unless the student will begin classes at the transfer school or program within 5 months of transferring out of the current school or within 5 months of the program completion date on his or her current Form I-20, whichever is earlier.

I am even more confused. I wanted to make an informed decision and it would have helped me so much to get the right answer but....... It's so hard to be an immigrant.

Your latest i20 Page 3 will have dates that list the OPT periods that you requested, if you did STEM extension that is also mentioned there. EAD dates will be same as the requested date on that i20. That is the vaild period of your i20, not the initial one listed on the first page, which is the estimated degree completion time.

Sometimes reading the documents you have signed helps.