Please give me help

How I can write a macro in excel for copy and paste a formula with
depending upon the value of next right cell

For example I have a formula in cell A1 to get numeric value from alpha
numeric value in cell B1 . That forumula I have to copy down from A upto
the data contained in next cell B . ie, that may to copy from A1..A10 or from
A1..30 that is depending the data of B cell

Please Help
With Thanks and Regdards
Pol