Our hope is that clients upgrade every second year.  That doesn’t always happen, but that is my suggestion.  If companies get too far behind, it becomes harder to support their system as there can be conflicts with other software  (which is also always upgrading).  Moreover, if the objective of the CRM strategy is to make better use of the information that flows through your company, holding back on upgrades… ends up costing the client more than the expense of applying the upgrades.