Forums on Squad might be your best source, or the squadhelp thread on here. Most dont give input when there are pre existing related threads.
my info :
55 premiums added over 6 months.
about 6 months till a sale.
2 sales total now. 1 is on payments.
as for any relation to knowing when people see first sales, has next to no comparison to what your experience will be like. some sell many , som sell none. its very unpredictable.
but what i can tell you is, they are by far the best option, there marketing is top-notch.