Having 2 numbers that guaranteedly have a point where they can be used to get any number, how can this point be estimated?

For example, I have the numbers 3 and 5 I know that I can get any number starting from 8 by adding diffrent combinations of these numbers ie 9 = 3+3+3 and 10 = 5+5. How, knowing that the numbers guaranteedly have this point (where I can get any number starting from x), how could I estimate what x is?

In that case wouldn't x = 1 ?  You can add as many 1's as it takes to get any number.  Any positive number.   

_.

no,

(where I can get any number starting from x)