My first two applications were less than 30 days apart.
I'm following 2 applications every 65 days. That is, my application dates are
d1, d2, d1+65, d2+65, d1+130, d2+130,...
So, in addition to the 1st and 2 apps being < 30 days apart, so are the 3rd and 4th, 5th and 6th, etc.
No problems yet...
I am pretty sure that there is no 1 app per 30 day rule.