You can't easily do 'alerts' directly in Excel, without awkward use of VBA. Some other options are:

a) Use VBA to automatically generate calendar appointments in Outlook
b) Use conditional formatting in your excel spreadsheet to highlight upcoming/overdue assessments
c) Filter your spreadsheet for upcoming/overdue assessment.

At a starting point, you will want need three columns for each contact - one with 'date of last assessment', one with 'period between assessments' and another that calculates 'due date for next assessment' from the previous two columns.