How can I foresee the time of computation of an algorithm?

I have an program that make combinations, if I give a small number of input... the time of computation is small!, if I give a large number of input, the time of computation may be long (days, months....)

I need to know how can I do to know how much time the computer will take to complete the operation.

regards,

I think you have some choice:
one easy way is to guess the number of operations you have to do for some number of input. Then, you have to know the time it takes to do n number of operations and then a small ( x ).

It's a very hard thing to do. Even Microsoft is not able to do it ... (yes, I know they lack of quality control but they have a lot of $)