I have a worksheet labelled Early Adopter Scheme whereby the cells in column
F contains a start date, column G calculates the number of months between the
start date and today(), and column L contain a SUMIF formula, referencing a
worksheet labelled OCR (in the same workbook). The data in the OCR worksheet
will be updated on a monthly basis.

I would like the SUMIF formula in column M to continue performing the
calculation where the value in column G (the number of months column) does
not exceed 12. When the corresponding cell in column G does exceed 12, i.e.
12 months, the end of the monitoring period is over and we are no longer
interested in any additional business

Is there any way of doing this? - allowing the calculation to perform until
the end of the period?

Many thanks