Event frequency

Hi, I'm having trouble finding a way to solve my problem. I have a worksheet that has multiple rows of clients information, I have columns for the months of the year and when a client requests our services I add a cross to the corresponding cell. I need to calculate how often each client needs our services, for example if a client called us january, april and august, in average he calls every 3 months that way we can know vaguely when to call them to offer our services. Hope you can help.

Thanks in advance.

Assuming that you have January in column B, December in column M, 1st client data in row 2, try this array formula in N2

=SUM((MAX(IF(B2:M2="x",ROW($1:$12)))-MIN(IF(B2:M2="x",ROW($1:$12))))/COUNTIF(B2:M2,"x"))

...confirmed by pressing CTRL+SHIFT+ENTER to activate the array, not just ENTER. You will know the array is active when you see curly braces { } appear around your formula. If you do not CTRL+SHIFT+ENTER you will get an error or a clearly incorrect answer.