Please can someone tell me how easy/difficult this is...

I need to create a routine whe

1. The values of a number of cells in my spreadsheet are built into an
equation.
2. The equation is run repeatedly until the value = 0
3. The information I need is the number of times it is run before value = 0

I have done this 'long hand' but would like it built into the function
contained within one cell. What are the key functions I need to be looking at
in order to create this?

Thank you.

James