H1 to F1 PhD, F1 years limit

[h1behelp]

Hi,
I'm currently on H1. Before I pursue PhD program. I want to make few things clear:

I heard there is 2 years slot of education during H1(6 years quota).
H1+F1= 8years.
Is there is any limit of number of years one can go to school when have/had H1?
Can I enroll in PhD which is normally for 3-4 years?

[txh1b]

There is no such limit for F1.

[kabkaba]

Once you change status to F1, it does not count towards your 6 years of H1, but you also cannot work off campus without authorization.